While only one person appears on most of the magazine covers, Billie Eilish is actually a duo — at least, musically speaking. Nearly every song the 17-year-old star has ever released was co-written with and produced by her 22-year-old brother Finneas O’Connell, whom she credits and affectionately introduces to concert audiences as “my big brother and my best friend.”

“She’s incredibly generous about it,” says Finneas, who performs under his first name. “It’s crazy and cool to me that people are so aware of the creative circumstance.”

While the Eilish phenomenon has been scrutinized ad infinitum — not least in this week’s Variety cover story, where Eilish is our Hitmaker of the Year — less discussed has been the musical partnership at the center of it. “All anybody wants to talk about is me,” Eilish says. “Nobody really talks about the music.”

That music is the end result of a long process that began when each of them was 12, attending a songwriting class taught by their mother, Maggie Baird, an actor and amateur songwriter.

“I always say it was actually the Beatles who taught them to write songs,” Baird says. “Because the class was for kids, I had to simplify it: ‘Here’s “I Want to Hold Your Hand,” let’s give these pieces of the song a name.’ The class was just an hour a week for 10 weeks or so — but Finneas was like [mimes ‘Eureka!’ moment], and in a few months he formed a band. All I had to teach him was the basics, and he immediately got it — and the same with Billie.”

Finneas had already been writing for several years when the pair first started working together, on a lark. “The first songs that we worked on, we didn’t write together,” Eilish recalls. “He wrote this song called ‘She’s Broken’ and I wrote one called ‘Fingers Crossed,’ and we recorded them and put them out on SoundCloud, just for fun.”

Then they recorded and posted a song Finneas had written called “Ocean Eyes” — which is where the phenomenon all started in November of 2015. The song, which has since racked up more than 30 million streams on SoundCloud alone (the official video has more than 180 million views on YouTube), quickly brought them not only thousands of new fans but also their management and label deals.

“We put that out and then somehow it got huge, and then things got super weird and very surreal,” Eilish says. “And then we started writing together — it wasn’t like a definitive decision, but we just started doing it. I think the first one was either ‘Hostage’ or ‘Bellyache,’” both of which appeared on Eilish’s 2017 debut EP, “Don’t Smile at Me.”

Now, Finneas estimates that “probably 75-80% of the songs are written with us sitting next to each other at a piano or with a guitar, singing a melody together,” he says. “It’s like a relay race — we really feel like we both have to kill our portion of it to get to the finish line.”

It’s a process the pair re-enacted during their recent Howard Stern interview, which found them vamping on a couple of chords while Eilish sang nonsensical lyrics. “We were just showing what writing looks like for us,” she says. “You know, playing random sh–, singing in tandem, making up words and singing words that aren’t even real words.” (An eavesdropping reporter heard similar sounds emanating from Finneas’ dressing room before Eilish’s October concert in Houston, where the pair were working on a song that in retrospect may have been their recent single, “Everything I Wanted.”)

While they learned the traditional rudiments of songwriting in their mom’s class, “the main idea was that once you know the basics, then you can change them,” Eilish says. The pair have gone to town on that concept — in fact, few of their tracks follow any conventional structure. “Bury a Friend” starts with the chorus, “You Should See Me in a Crown” has a middle section that’s just a single bass note, in several songs it’s not completely clear which section is which. Eilish comically trips herself up trying to outline the structure of “Bury a Friend”: “Chorus, verse, prechorus, hook verse, bridge, verse, chorus — no, other bridge, then hook… I don’t know,” she laughs. “It took us months to figure out what was going on there, but it works — like, it doesn’t, but it does.”

Like all great songwriting partnerships, the pair bring very different yet complementary elements. Asked whether she brings dark and offbeat ideas to Finneas’ more conventional, musicianly work, similar to the songs on his recent solo EP, “Blood Harmony,” Eilish initially dismisses the concept. But a few minutes later, she pipes up, “You know what?,” she exclaims. “I was just thinking about the question you asked, about him having a more classic and straightforward songwriting and mine being kind of darker — it’s actually really interesting because it’s true, and I’d never thought about that. I think it’s because he is really good at writing a verse-chorus-verse-chorus-bridge core, and I’ve grown up listening to and really liking songs that have, like, no structure, like Frank Ocean or Imogen Heap — songs that kind of don’t make sense, but they do. I actually love that, because it’s another way that we get things right.”

Yet at the same time, Eilish brings a classic, old-school pop flavor to earlier songs like “Idontwannabeyouanymore” and “My Boy” that many critics have cited as Amy Winehouse or Lana Del Rey influences, but actually goes back to the 1950s singers her parents played at home. “That’s also part of how I was taught,” she says, “I used to listen to tons of old music — Frank Sinatra, Peggy Lee, Etta James, Johnny Mathis and people like that — where all the songs had a perfect songwriting structure.” And while her and Finneas’ work is unconventional, it’s just as labored-over. “We’re pretty much perfectionists, even when we’re intentionally not following ‘perfect’ song structure.”

Given their schedules, the pair find time to listen to a remarkable amount of music — Eilish lugs around an extremely heavy backpack with two big speakers inside that she rocks with her playlists at every opportunity. Asked what she’s been listening to lately, she pulls out her phone and cites British singer Elouise, singer-songwriters Banks and Sabrina Claudio, female rappers Tierra Whack and Melii along with older artists like Avril Lavigne, Christina Aguilera, En Vogue and TLC. Finneas lists the latest albums from Post Malone, James Blake and Dominic Fike along with “revisiting” Chris Cornell’s work and Coldplay’s “Viva la Vida” alongside influences ranging from rock bands like Green Day and Imagine Dragons to singer-songwriters like Ben Folds, Duncan Sheik and Glen Hansard.

Yet what may be most remarkable about this unusual partnership is that at a time when most hit songs are created by songwriter-producer committees that can number a dozen or more, Eilish and Finneas write nearly all of their songs themselves, with Finneas producing. It was a hard-won process, as their label and management initially encouraged them to work with more-experienced songwriter-producers.

“Who would have believed that Billie’s [then-] 18-year-old brother would be her only cowriter and producer, and that he’s as good or better than anyone else?,” Baird says. “They were not pushed to do it by the label or their managers, but they did say ‘Let’s just try it and see how it works.’”

Eilish is less diplomatic. “My managers and label are so incredible and I’ve been so lucky with my team that I think I’m pretty spoiled,” she allows. “And it wasn’t like they were forcing us into it. But that was so stupid. It was horrible for a lot of reasons. I was a little tiny teenager who hadn’t written with anyone besides my brother. And there I was, 14, in a room with 40-something-year-old dudes who were trying to write a song with me and Finneas, who was 18. I didn’t want to be doing that! Jeez! Stuck in a room with a bunch of adults, trying to write a song? Gross!

“But we tried it, and I’m glad that we tried it,” she concludes, “so we knew that we didn’t want to do it. And also, all the songs we wrote with [others] were not as good as the songs me and Finneas wrote alone.” (While reps demurred from identifying the collaborators in question, two songs with non-O’Connell family writer-producers were released last year, “Bored” and “Bitches Broken Hearts,” the former with Banks/ Donna Missal collaborators Tim Anderson and Aron Forbes, and the latter with singer/songwriter Emmit Fenn.)

Despite her daughter’s vehemence, Baird is more positive about the outside collaborations. “I actually thought a lot of those songs were good, although Finneas says I like everything they do,” she laughs. “And they’ve actually done a few sessions where they wrote amazing things with other artists, not a [songwriter-producer for hire] — Jessie Reyez, Vince Staples, Khalid — and Finneas was the producer.”

Eilish waxes ecstatically about a song she wrote with Reyez, which she calls her favorite that she’s ever written (at least, on this particular October day). “I met her at a concert and we both had off-days, and somebody was like ‘We should get you guys in the studio,’” Eilish recalls. “Normally that sentence makes me want to puke, but I was like, Sure, I love Jessie, she’s incredible, and it’ll be fun to be in the studio with her and her energy, even if we don’t make anything.”

However, once they got there, “I had this crazy amount of writer’s block, and she and Finneas were both coming up with stuff and I was just sitting there with nothing and felt like a big loser,” she continues. “But then we took a five-minute break and Finneas started playing this beat, and suddenly I just came up with this melody and all these lyrics, and then I wrote this whole song and we recorded it — it had never happened that fast for me.” Although she calls it her “favorite song and the one I’m most proud of,” it remains unreleased.

And although “Everything I Wanted,” Eilish’s ethereal new single, is more low-key than the doomy bangers on “When We All Fall Asleep,” it doesn’t seem like an obvious signpost for the direction their songwriting is headed.

The two of them don’t offer much concrete direction, either — when asked, Eilish can’t even summon an adjective. “The music we’re making right now, like …” she begins. “It might be a little bit more …” Long pause. “It’s not even, it’s … I can’t even describe it, because it’s …” she tails off.

Finneas also stops short of a straight description, but offers some entertaining metaphors. “The way that we tried to approach every piece of music is, if the song had a brain, it would be aware of its catalog,” he says. “Like, the [‘When We All Fall Asleep’] album knew that the [‘Don’t Smile at Me’] EP and those other songs exist. They’re referential in certain ways: There are lines in ‘Crown’ about ‘Ocean Eyes’; ‘Ilomilo’ references ‘Bury a Friend,’ which references ‘Xanny.’

“It’s a little bit like a book series,” he continues, warming to the theme. “Like, for ‘Harry Potter and the Chamber of Secrets,’ J.K. Rowling had to figure out what happened after ‘Sorcerer’s Stone,’ and for us, album two will be like what happens next to the person who went through the first record.”

Finneas has parlayed his recent success not only into a budding solo career — for which he’s being hotly pursued by several major labels — but also work with other artists: He co-produced Selena Gomez’s recent single “Lose You to Love Me” and collaborated with Camila Cabello on a song from her forthcoming second album, which comes out Friday. “I saw [Cabello’s] set at Lollapalooza last summer and texted my management, ‘I want to work with Camila more than I want to work with any other pop artist right now,’” he enthuses. “I love her voice, I love her tonality. I love the way she’s intimate in her lyrics, and she puts on a great show.” He also worked with John Legend “on a song that I don’t think will ever come out” but called it a “master class in songwriting,” and acted as an executive producer on a pair of EPs by female singer-songwriter Ashe.

Yet anyone seeking an updated version of “Bad Guy” won’t get it from Finneas. “As soon as you make anything that people like, you get all these new artists hitting you up like ‘I want to sound just like Billie Eilish,’” he says. “And I’m always like, ‘Absolutely not.’ I’m interested in doing things in the pop world that are as far away from Billie as possible, and have them stand in stark contrast. That’s what’s great about working with artists like Camila — she comes in fully realized. I would love to have a career full of albums with a variety of artists and have no throughline to them at all, and have people say, ‘I love this, who produced it?’”

And although Finneas is energetically pursuing a career outside his work with his sister, he’s fine with his role as copilot of the Billie Eilish rocketship. “I’m not very interested in fame or notoriety at all — in fact, I’d be pretty bummed out if I woke up one day and I was, like, super, super famous,” he says. “But the flipside of that is that I’m really passionate about my music, I’m really proud of it and I want it to be heard by as many people as possible, and I’m willing to embrace whatever comes with that.”