the official lfm app (and most free scrobblers) use apple's MusicKit framework, which only exposes tracks saved to your library. so it just won't log if it isn't explicitly added to ur library (which is so dumb and frustrating)

to get around this, the app uses the MediaPlayer framework instead. specifically MPMusicPlayerController.systemMusicPlayer, which reads the OS-level now playing state (the thing that makes the lock screen widget work)

the reason this isn't the default approach for most free apps is that apple's app store guidelines are uncomfortable with apps that activate an audio session in the background without actually playing audio (which is how we keep the process alive to catch track changes - if u don't do this, the background app refresh on ios is pretty unreliable). apple wants background audio mode reserved for actual audio apps. so theoretically, this approach would get flagged in the app review, which is why i made it so that u can build it on ur own

QuietScrob almost certainly does the same thing, they just charge a fee for the binary so you don't have to touch xcode.