I can’t answer the rest, but for the “how”: iOS has a “currently playing” API just like macOS and Linux (via MPRIS). Individual players need to support it, but

I can’t answer the rest, but for the “how”: iOS has a “currently playing” API just like macOS and Linux (via MPRIS). Individual players need to support it, but any app with the right entitlements should be able to read from it. In other words, this particular case isn’t that nefarious: Spotify is publishing the “currently playing” metadata so that other apps can read it, and Apple is consuming it.