Paul Mills is using his phone to live-stream footage of his twin sons while they record their first music video. Max and Harvey Mills, who are 13, are lip-synching to music by Shawn Mendes, the Canadian singer who got his break on the now doomed Vine video app (two albums later, Mendes, 18, is preparing for his second world tour). “Bleedin’ till I can’t breathe, shaking, falling on to my knees.” The brothers mouth the words while Raja Virdi, a young videographer who has worked with Adele and Sam Smith, moves around them with his camera.

The boys stand next to each other in the skate park under the Southbank Centre in London. They wear matching black skinny jeans, bought for them by their mother, Sara. Max, who strums his guitar, has an H&M bomber jacket, black beanie (Max always wears hats, to set the twins apart) and his prized white Nike Air Force hi-tops. “These are probably the only things that are, like, show clothes,” he says of the trainers later. “I wouldn’t normally wear them outside.”

By 10am on this sunny morning during the half-term holiday, Paul’s phone is already running out of battery. “Right now, there are 11,732 people watching this,” he says, reading the number in one corner of the screen. “That will accumulate.” Comments left by fans move up the bottom of the screen almost too fast to read. “Please follow me u are so good,” says marvelnick. “Love u,” says Antonia_bayramxox, who adds 28 heart emojis. I move in front of the camera and introduce myself to the invisible crowd to ask why they love Max and Harvey so much. The comments speed up. “There music!” says Jxtie. “Cuz they’re so cute and talented,” another fan says. “They’re amazing in every way,” say Finnandolivia. Brookea_xx turns the tables: “MAX AND HARVEY PLZ ANSWER THIS QUESTION: will u ever be in Surrey??”

Emojis from fans bubble up from the bottom of the screen in a fizz of cakes, pumpkins and cherries

Xtabbycatx stands nervously by the railings beside the Thames, watching the recording. Half an hour ago, the 12-year-old was having breakfast at a hotel a mile away. She was scrolling through Instagram, where Max and Harvey have almost 250,000 followers, when she dropped her toast. “I saw their picture and they said they were here and I just freaked out and left breakfast and dragged Dad out,” says Tabitha Wilson, the girl’s real name. She and her dad, Darren, 50, are visiting London from Stafford. She was hyperventilating as they crossed Blackfriars Bridge, Darren says. But why? “I watch them all the time, and they just make me happy,” his daughter explains. “They’re funny and they’re really good singers and they just make me smile, and I wanted to see them.”

The digital adoration is still draining Paul’s battery. Next to the comments, emojis from fans bubble up from the bottom of the screen in a fizz of cakes, pumpkins and cherries. Another number keeps count of these “likes”. When Paul ends the broadcast after 35 minutes, fans have deposited 1.96m emojis. Almost 100,000 people have tuned in at some point, a figure that would delight most TV channels at this time of day. Paul plans to do more live-streaming later, “when America wakes up”. But first he needs to charge his phone.

Facebook Twitter Pinterest The twins have 2.48m followers (and gain about 10,000 new fans a day).

Photograph: Mark Chilvers/The Guardian

Max and Harvey are Britain’s biggest Musers, the name given to the users of Musical.ly, an insanely popular social music app. Launched in Shanghai in 2014, it began to take off last year and has quietly hit the phones of more than 140m users. That’s almost half the number of people who use Twitter, which took three years to build up a comparable following. At its simplest, Musical.ly is a music video maker combining features of Snapchat, Instagram and Vine. Its founder, Alex Zhu, came up with the idea after spotting teens on a train filming themselves listening to music on headphones.

In the app, Musers choose from a library of more than a million songs, select a 15-second snippet, then lip-synch to it exuberantly while filming themselves, typically in selfie mode. Increasingly, people perform comedy skits, dances or lip-synchs to film dialogue. Some, such as Max and Harvey, sing for real as well as lip-synch. They can add filters and Snapchat-style face adornments (selecting “swag” adds a gold chain and a black cap). Musical.lys, as the looping clips are known, are then shared with followers. Live.ly, launched in June, a live-streaming sister app, gives Musers the freedom to broadcast anything, for 20 minutes at a time on average.

The twins lead a tribe of 2.5m Musers in the UK, most of them aged 13 to 24. At the time of writing, their joint account has 2.48m followers (they gain about 10,000 new fans a day). They never drop off the top of the UK leaderboard, and routinely feature in the global top 10. In the US, home to half of all Musers, the biggest stars have more than 10m fans. Musical.ly, which has about 120 staff in Shanghai and Los Angeles, estimates that half of all American teenagers now have the app. Teen by teen, it is holding up a mirror to a new generation of digital natives, and changing the music industry.

Facebook Twitter Pinterest Max and Harvey’s mashup of Treat You Better and Stitches.

Musical.ly features videos it likes on the app’s homepage; the sudden exposure means Musers can get very big, very quickly. The app’s biggest stars now have agents, merchandise and, in some cases, record deals. It’s why Max and Harvey, who signed up in March, are making a real-life music video, with their own voices – they want to take their success to the next level. Today’s shoot, and a session in a recording studio yesterday, have been arranged by Cassie Petrey, who runs Crowdsurf, a digital marketing firm in Los Angeles, and Tim Byrne, the British former creative director at Syco Entertainment, Simon Cowell’s label (Byrne now runs Fandom Management, also based in LA).

When Petrey was 12 and at school in Kentucky, she made an online Backstreet Boys fanzine. Years later, while working for record labels in Nashville, she alerted executives to a new website called Myspace, and ran pages for artists. Now 30, but “still a fangirl at heart”, she looks for talent on Musical.ly. She says the app is changing the game. “Max and Harvey did a Live.ly a couple of days ago, and there were, like, 50,000 people watching it at once,” she says by phone (the boys were fooling around at Alton Towers). “I’ve never seen live-streaming figures like that on any platform.” Musical.ly also has a uniquely young audience, which partly explains the sense, outside the app, of a quiet rise to prominence. “I see kids commenting all the time that they can’t follow talent on other platforms, because their parents only let them use Musical.ly,” Petrey says.

Max and Harvey had about 150,000 followers when Petrey spotted them. She says she knew immediately that they could be stars. Her job now is to work on their digital output beyond Musical.ly (they need no help there). They now have almost 10 social media profiles, including a YouTube channel (3.9m views and counting). Engagement is the buzzword in this world, and Musical.ly succeeds by appearing to blur the line between idols and idolisers. Popular Musers are part of the community that sustains them, and a couple of featured clips can be all it takes for anyone to make it big. Really big.

Facebook Twitter Pinterest Baby Ariel: ‘Before social media, I didn’t know who I was or what I wanted to do. Now I do.’ Photograph: Paul Zimmerman/Getty Images

First came YouTube, where Justin Bieber put up his first video almost a decade ago, then Vine. “Now people are saying, ‘I want to be a Muser’, and we have the resources to make that happen,” says Tiffany Au, 28, director of communications at Collab, a “digital talent network and entertainment studio” in LA. The firm helps about 400 digital creators, including Ariel Martin, better known as Baby Ariel. She is from South Florida, has just turned 16 and is by some distance Musical.ly’s biggest star with – let’s just check the latest figure – 15m fans. She got the app a little over a year ago, after a friend showed it to her. In those early days, Musers were uploading static lip-synchs. “I was like, how do I make this interesting,” Baby Ariel says, “so I decided to use hand motions and thought, oh, this is actually pretty cool.”

She has turned phone holding into an art form, and has been imitated by millions of Musers. In one recent clip, she lip-synchs to the Chainsmokers’ hit Closer (it ambitiously rhymes “back seat of your Rover” with “tattoo on your shoulder”). Turning her phone into a choreographic aid, she makes it move around her face in time with the music, like a hummingbird sizing up a flower. While she chews gum through an exaggerated smile, mouthing the words “so baby pull me closer”, she uses her free hand to draw the camera in. She says her clips can take between 20 and 90 minutes to create.

Before long, Baby Ariel was getting recognised. “This girl came up to me in Old Navy and I was like, ‘No way, this is crazy. This is not a real thing’.” She later left school to study online, so that she could devote more time to Musical.ly, alongside her mother Sharon. She is now a “social media influencer” with aspirations to act. She speaks to me from LA between auditions at Disney and Nickelodeon. She is also in town to join Jacob Sartorius on his US tour. Sartorius, 14, has more than 12m fans, and now records original music. Sweatshirt, his first single (“Girl, you can wear my sweatshirt, ’cause you’re the only one I hold, And I don’t want you to be cold”) made the iTunes top 10 and has more than 35m YouTube views. At the Troubadour club in Santa Monica, where, in 1970, Neil Diamond introduced Elton John to America, Baby Ariel DJ’d while Jacob Sartorius of Oklahoma performed Sweatshirt to a sellout crowd.

Fans are desperate to meet star Musers – and each other. When 17-year-old Violet Summersby noticed this in comments, she arranged a meet-up in New York. A dozen or so big names mingled with about 300 fans. Fifteen free events followed in cities across the US. Now the co-founder and CEO of Muser Movement, Summersby hosted the first ticketed meet-up in LA in November. As well as encouraging engagement, she believes the app reflects a healthier relationship between young people and social media. “It doesn’t matter what you look like,” she says. “If you’re funny or you’ve got something to say, you can put it out there. Social media is changing now. There was this idea that you had to look like Kim Kardashian to be famous. That’s not the case any more. You can be whoever you want to be, and that’s now acceptable. It’s not acceptable to be something that you’re not. People want to see your true self.”

Facebook Twitter Pinterest The twins film their new video.

Photograph: Mark Chilvers/The Guardian

It’s a stirring vision, but it doesn’t immediately square with the high proportion of telegenic, zit-free teens among Musical.ly’s upper ranks, or with concerns about safety and privacy on the app. It is also clear that a significant proportion of Musers are younger than 13, the minimum age for app store downloads. There have been sporadic reports of attempted grooming via the app, and concerns about inappropriate lyrics and content; at least two UK police forces have reportedly investigated complaints by parents. But Musical.ly encourages parents to monitor use of the app, and says it removes inappropriate content within 15 minutes; it adds that concerned Musers or their parents should consider sacrificing potential fame and make their accounts private. All those I speak to, however, report seeing less nasty stuff, if any at all, on Musical.ly than elsewhere. Moreover, even if the top Musers resemble the glossy Mickey Mouse Club stars of yore, they believe that, for now at least, Musical.ly offers a freedom of expression and control that would have been alien to Britney Spears. “Musical.ly has allowed me to find who I am as a person,” Baby Ariel says. “Before social media, I didn’t know who I was or what I wanted to do. Now I do.”

Six months ago, no one would have ever approached us unless they were asking for directions

Where Musers find freedom and community, the music industry sees dollar signs. All the major labels make their tracks available on the app, which mobilises a bedroom battalion of promoters. In August, the label Republic reported a 23% increase in sales of Hailee Steinfeld’s Starving after a promotion on Musical.ly. In the UK, Olly Murs of X Factor fame launched a #Challenge, another popular feature of the app, to promote a single on his new album 24 HRS. He lip-synched to Grow Up while wearing face filters, including a teddy bear’s nose. “Create your own ‘Grow Up’ lip-synch video & you could win a signed copy of 24 HRS,” he wrote under his Musical.ly. Thousands of Musers have done so, adding the #GrowUpChallenge hashtag to their own clips and multiplying Murs’ audience. At the time of writing, Max and Harvey have made the most-liked clip.

The established stars courting Musers are at risk of being outshone. Baby Ariel now has almost 2m subscribers to her YouTube channel, roughly the same as Ariana Grande, Selena Gomez and Shawn Mendes, perhaps the biggest conventional singers going for the same demographic. “We had an event at Disney, and all these Disney stars were there on the red carpet, but all the fans were around Baby Ariel,” says Alex Hofmann, Musical.ly’s German-born head of operations in North America. In October, Ariel made the cover of Billboard magazine. Thanks to Live.ly, the live-broadcast app, Musers are also cashing in. Those who stream footage through the app can receive special gift emojis that fans buy for cash as in-app purchases. iTunes and Musical.ly get a cut; the rest goes to the recipient, who may reward the gift with a BFF (best fan forever) award. According to the company, the top 10 Live.ly users have each made $46,000 on average over a two-week period. “People are quitting jobs to do it full-time,” Hofmann says.

Facebook Twitter Pinterest Max and Harvey with superfan Milly Fincher.

Photograph: Mark Chilvers/The Guardian

Back on the banks of the Thames, Max and Harvey have moved location to the London Eye. Another superfan turns up. Milly Fincher, 13, is shaking with excitement. She had been watching the skate park Live.ly at home and begged her dad, David, to bring her into London from Surbiton. During a break in filming, she meets her heroes and is speechless. They greet her like pros. “So your name’s Milly, yeah?” Max says. She nods. “Awesome. How old are you?” She tells them. “Awesome. Would you like a picture?” They pose together. Milly posts the picture to her Instagram later: “So this happened, and I’m gonna be in a newspaper with them!” she writes, adding five screaming emojis.

“It’s still crazy, for sure,” Max says while considering fame on the way to Wagamama for a lunch break (they wanted Nando’s, but it’s a bit of a walk). “Six months ago, no one would have ever approached us unless they were asking for directions,” he adds, while Harvey plays Pokémon Go, occasionally glancing up to check for obstacles. “Now, people are asking people for directions for where to find us.”

Facebook Twitter Pinterest A compilation of this year’s Max and Harvey’s musical.ly videos

Fame has started to change family life, even if the money hasn’t – yet. The boys’ PO box address receives about a dozen items a day, including chocolates, fan fiction and artwork.

I ask them if Paul is secretly a pushy parent. He used to be a singer himself – musicals, mainly – and met Sara while performing on a cruise ship. He gave it up to support a family that also includes two younger children, and now works for a double glazing company. Max and Harvey laugh at the question. “Help me!” Harvey mouths silently, pretending to be a hostage.

“Part of me thinks I’d love to pause this now and come back to it when they’re 18, and had a bit more of a chance to grow,” Paul says.

But the boys are way ahead. I ask where they’d like to be in five years, at 18. “Well, that’s probably when we’d like to be touring globally,” Max says. Paul almost chokes on his noodles.

“Shush, Dad!” Max says.

“Yeah, then five years later, we’ll run for president,” Harvey chips in.

“Yeah, then we’ll build a massive wall,” his brother adds.

Where would they build it? “Erm, between me and my brother in our bedroom,” he says.

Now all three Mills boys are laughing, before Virdi, the video guy, checks his watch. It’s time to go back outside and film the last shots.