The 19-year-old singer has teamed up with Rosalia on 'Lo Vas A Olvidar' ('You Will Forget Her') and though they have only just dropped the track, it's been in the making for almost two years and had originally planned to release it last summer.
Billie told Apple Music's Zane Lowe that they originally wrote the first verse up until the chorus in January 2019 before returning to the track six months later, with the 'Con Altura' singer adding the second verse.
The 'Bad Guy' hitmaker continued: "We didn't get back to it until literally last year, 2020. And so that was mid-quarantine. We're just like, 'Yeah, let's work on this song.' Because in our heads, we were going to put it out in the summertime.
"I just was like, 'It needs something else... We wrote it was the whole song was done, except we needed this last line."
The final line came when Billie and her brother and collaborator, Finneas O'Connell, along with his girlfriend Claudia Sulewski, attended one of their regular Friday dinner and movie nights with their parents Maggie Baird and Patrick O'Connell and decided to play them the song.
She recalled: "We all came down to the studio, and we listened, and me and Finneas were like, 'Should we just write this line right now?' So I was sitting in the room with my entire family.
"We both, all of us, including my dad and my mom, they were like, 'What if it was like...' We were all giving ideas.
"So technically, they should have a writing credit, because we all wrote that line right at the end."
The tricky line in question is: "You say it to me like it's something I have any choice in/ If I wasn't important, then why would you waste all your poison?"
Meanwhile, Billie admitted working with Rosalia was a whole new experience for her.
She said: "It's so not something I feel like I've done. Rosalía opened a little channel in me that I hadn't tried out before."
And the 'No Time To Die' singer thinks it was "so important" that a lot of the track is in Spanish.
She said: "When were writing the song, I remember her saying something about, 'It should be in English.' And I was like, 'No, no, no. It should be in Spanish. It's so beautiful.'
"I like tons of music from languages I don't even speak... I've been really intrigued by it pretty much my whole life."