^ I wouldn't say they're mainstream in the west but they're becoming bigger and bigger, it's mostly thanks to fans but also their fanbase keeps growing more and more, and it kinda looks like once someone from GP starts liking them they eventually turn into an ARMY haha
They get a bit of radio airplay in the USA, as well as interviews (including Ellen DeGeneres show, Jimmy Kimmel, James Corden), they've attended a couple of Awards shows in the USA... A lot of US artists expressed interest in them (whether it's genuine or just for clout we'll never know), so I'd say they're still phenomenal even though they're not "mainstream".
I can't say whether all spotify/youtube streams are coming from ARMYs only, or also from GP too though, but even so it's still thanks to fans since they were the ones to push BTS so hard in the first place, back when BTS used to be nobodies even in Korea.
But that's the USA. UK is a different story
They don't get as much recognition here, radios still seem to ignore them (moreso than in the USA). There were a couple of radio stations playing DNA and Fake Love. And also BBC Radio 1 played IDOL, like, 1 or 2 hours after it was released.. But also despite them charting at #21, MTV didn't air their music video when announcing the Official Singles Chart ranking because "Unfortunately there's no official video for Idol, so we're moving on to the next spot" lmao
Speaking of the UK, I saw the BTS x Puma collab shoes in store at the Puma flaghip store in London yesterday ;-; And they've been showing the LG x BTS ad at Piccadilly circus for, like, 2 months already. They're selling the BTS albums at HMV too, not only in the kpop/world music section, but I've also seen Tear and Answer in the Pop/Rock section and the "New & Trending" shelves too since, like, June or something lol