I paired via bluetooth my phone (Moto G5, Android M stock non rooted) to a linux box, and among the services that it advertises I found out that the phone can act as an AVRCP remote (that is, send play/pause requests etc just like a bluetooth headset with buttons would do). In linux (which uses the BlueZ stack) this is automatically handled by creating a virtual keyboard that can generate events such as volume up and whatnot, so the receiving side is working. However, I cannot find a way to actually send these commands from the phone. Using the bluetooth volume slider doesn't seem to do it (it must be a knob of the software mixer on the phone).
Is an app necessary to use this functionality, even though the service is always advertised by the phone? If so, which one?