"It's like I'm surfing for the first time and a big wave just came through," she says.
"And I'm trying my best to get through it."
In the film, Golden is performed in the film by girl band HUNTR/X, consisting of Rumi, Mira and Zoey.
According to the Official Charts Company, it's the first K-pop single to top the UK chart since PSY's Gangnam Style in 2012.
Ejae, who provides the singing voice of Rumi, performs some of the soundtrack's songs and also co-wrote Your Idol, How It's Done and the Hunter's Mantra.
She tells Newsbeat ending up on the final recordings was an "organic process".
"I've been singing all the demos while writing the songs," she says.
"I guess we got used to me singing it. And they were like: 'Hey, do you want to sing?'
"I was just like: 'Why not?'"


