The approach
Native Kotlin, no game engine, and as few dependencies as possible: every library added has to justify itself in one sentence. That is what one person can understand, fix and maintain.
No tracker and no player profile, in the game as on this site, which has no cookie; the game only counts, anonymously, what happens to the artists’ songs, to report it to them. The game is free and shows an ad between songs, never during a game: Google AdMob serves it based on the phone’s advertising ID (in Europe, you first choose whether it may be personalized), and a one-time “no ads” purchase removes it. With an account, a shop also offers cosmetics, paid for in wubs, the game’s virtual currency, earned by playing, or in VIP tokens, which are bought: they only change how things look, never the score or the leaderboards. Its external services: Firebase, for the account, the Jukebox, leaderboards and progression; Google AdMob, for advertising; Google Play, for purchases and for local play between nearby phones; MusicBrainz and the Cover Art Archive, for song covers (the artist and title only, and the setting can be turned off).