/jp/ - Otaku Culture

Regarding how the doujin music on Spotify came to be:
Circles can sign up to touhou-music.jp to be part of the official music distribution service to platforms such as BOOTH (for digital download specifically), iTunes, Google Play Music (now YouTube Music), Amazon Music, NetEase Music, and Spotify.
I'm part of this group, so even my stuff is on Spotify now.
As to why your favourite circle isn't on there? They likely never applied to it as they are inactive/defunct now or simply prefer to print physical (e.g Eurobeat Union with half of its tracks being originals, which puts them in a messy spot here)
As for foreigners getting in? They're currently invite-only and even when available, you'd have to write emails in Japanese and get a Japanese bank account for revenue sharing.