From what I recall your app gets killed when executing in the background unless you actually play some audible sound through an audio session, but Spotify can somehow stay alive while being used as a remote control app even though it's not visible, doesn't play audio, doesn't track location, etc. It stays reactive to volume changes and server communication while being backgrounded. How?
Spotify implements their own network based cross-platform remote control of their apps.
For some reason I cannot find much info on background modes to shed light on what they could be doing. I can't find a documented use case that includes remote control.
For those who doubt that Spotify vends its own remote control solution I disabled Bluetooth and WiFi and kept using it as if nothing had happened. This is not about AirPlay, Bluetooth, etc.
iOS 13