Sometime between 2016 and 2017 – a distinct period, if you follow the rapid hot and colds of social media platforms – 16-year-old Austyn Tester filmed a video just for himself. Shirtless in front of a wall in his Kingsport, Tennessee, home, Tester flicks his hair with barely contained excitement. After months of live-streaming his middle American high school life on YouNow, cultivating a following of teenage girls who tuned into him lip-syncing, dancing or just hanging out, Tester received interest from a talent manager. Clutching himself and spinning around, he tumbled through a series of revelations. “This is before I’m famous!” He leaned into the selfie lens. “It is 10.06pm. I’m not famous right now. Hopefully I’ll be famous soon.”

An iPhone, propped up by a tip jar of Tester’s fan mail, plays this video midway through Jawline, a new documentary that takes an empathetic look at the vertigo of social media stardom, the always-on demands of late-2010s fame, and the newest generation of teenage fan. In the film, which won a special jury award at this year’s Sundance film festival, Tester offers a case study of this specific brand of fame, baffling to those not steeped in Vine, YouTube, YouNow and TikTok. Sixteen years old, with a southern drawl that can’t decide whether to stretch “fudge” or “fuck” (result: “fuuudgeya”), Tester wants to be famous, if only to escape his small town. So he turns to livestreaming, in which he broadcasts daily updates of his life and chats with teenage girls eager to have a boy tell them, earnestly, to chase their dreams.

By the time the director Liza Mandelup’s cameras arrive in Tester’s home – dishes stacked in the sink, old driveway toys still piled outside – Tester has already amassed a following of a couple thousand. His home life reveals social media’s dizzying juxtapositions: at fast food joints or jumping into a river he is a normal teenage boy, friendly and unremarkable. But in a chat room, his mere presence melts girls to tears. Linking the two identities is his camera – the online conduit to a shapeless fandom, as amorphous and precarious as a string-wand bubble.

While Tester filmed his potential last moment of pre-fame, Mandelup was deep in the scene of star meet-and-greets, where social media celebrities host fans for surreal performances of selfies, hype-manning and occasional music. In 2015, Mandelup had been reflecting on her own teenage pendulum of emotions when she discovered Magcon (an acronym for Meet-and-Greet Conference), the OG tour of Vine stars. It was “the strangest world I have ever seen”, Mandelup told the Guardian. “I just became obsessed.”

In particular, she fixated on how teenage fandom “must be so different today, to go through all the emotions of being a teenager when you have so much screen interaction, and so much of your life is lived online”. Moreover, she said, the intensity of teenage fandom – long dismissed, and made more intense by the omni-accessibility of social media content – was rarely discussed, let alone treated seriously. So Mandelup turned her camera on the cameras, starting with Fangirl, a short film on five unashamedly devoted teenage girls.

But Fangirl barely scratched Mandelup’s itch to unpack this latest, intense iteration of teenage girl fandom. Mandelup, who remembers the paralyzing, overpowering effect of seeing Leonardo DiCaprio on screen as a teen, understood, and began looking for an entry into the pressure cooker of hope and cynicism that is social media stardom. After a couple of false starts, she realized the best window to social media fame was “someone who was naive to how it worked, and just the total opposite side of the business – where they genuinely felt like I, alone with access to the internet, can become somebody”. A year into filming, she met Tester, and flew to his home in Kingsport.

Though it primarily follows Tester’s experience with fame, and is named for a particular facial feature beloved by fans, Jawline looks at the whole body of social media stardom – its fans, talent, and the mechanisms underlying them. Mandelup’s film treats each neutrally – which is to say, in a world of hot takes, empathetically – capturing each in a finite moment of fantasy. Tester dreams of escaping Kingsport, and boredom; teenage girls explain how their social media crushes provide an escape from real-life bullying and adolescent blues.

Austyn Tester in Jawline. Photograph: Hulu

It’s Michael Weist, a twenty-something, Juul-toting talent manager, who provides the dose of brutal realism on social media’s star machinery. He orders clients to post another video, follow this script, get this verification badge. While Tester celebrates taking home $46 from YouNow, Weist takes mini-vacations from his phone on Rodeo Drive. Describing the recent swell of influencers as a “social media gold rush”, Weist is frank about his priorities; his favorite part of managing social media stars, he says at one point, is his bank account.

“Looking back on everything in this film, it’s like a time capsule for me. And I have grown and changed immensely,” he told the Guardian in reference to that comment. But he maintained that “the reality of this is: why do these kids want to get famous? It’s probably not just so they can make a fan feel nice. It’s probably so that they can pay their mom’s rent, or buy a nice car, or something, if we’re all frank about it.”

Capitalism, and the players attracted to a profit by any means of attention, will not keep its hands off stardom – not least while the mechanisms, despite all appearances of authenticity, remain transactional. “There’s a tradeoff happening, whether people acknowledge it or not,” Weist told the Guardian. “An influencer is providing affirmation, validation, and that follower is getting something from them.”

Ultimately, however, the primary subject of Mandelup’s film remains her first: the teenage girl fan, a role Mandelup, a former moody teen, takes personally. “I had all of the emotions, like, ‘I feel so lonely, I don’t know if anybody in this world cares about me, should I even exist, I feel like nobody really knows me the way I wish they knew me,’” she recalled. From the Beatles to Leo to One Direction, there’s a long tradition of pinning adolescent hopes on star images. But as Jawline shows, the difference now is the steepening simulacrum of intimacy: how the mechanisms and “just-like-you” image of social media fame mimics genuine connection and access so thoroughly that it could be mistaken for the real thing.

Austyn Tester in Jawline. Photograph: Hulu

In the past, DiCaprio was in movies or in tabloids, not livestreaming on the phone in your pocket or broadcasting his breakfast while you’re at school. Now, Mandelup said, boundaries get mixed up. It’s easy to grow confused when you can say, “I wake up with them in the morning, I know what they ate for dinner last night, I know his little sister, I know his dog,” she added.

It’s a potent intimacy, she said, but inherently fleeting, as Tester, now 18, knows. After a modestly successful tour with other social media stars, he is back in Kingsport, working at Starbucks, and preparing to start community college this fall. And though he still posts to social media, he’s ambivalent about chasing fame and his status in the influencer ecosystem. “I definitely have a following on social media, but I would not say I’m famous,” he told the Guardian. As for the future, he said he’s not interested in fame for fame’s sake, as he was at 16. “I would like to say I’m more authentic now, I’m more original.”

It’s too soon to tell whether Tester’s brush with fame was a trajectory or a blip – social media is, according to Weist, both cyclical and unpredictable. But Mandelup sees the devoted, niche cults around social media stars as the unexpected confluence of the attention economy, unintended scale, and good old teenage emotion. “Thinking of it as a glitch is the thing that makes it make the most sense to me,” she said. “I don’t think anyone in society intended for it to go down like this.”

Jawline premieres on Hulu on 23 August and in the UK at a later date