Make Apps' Volume control system Volume

The concept of having an Apps’ volume and a system volume was one of the first that Android removed.

Thus, many people find it strange.

For viewing movies I found Celluloid (Flatpak) to be the best of all. Haruna could surpass it, if some basic issues are dealt with.

How could the app change the system volume, i.e. mirror the system volume on it’s volume slider?

Would this need a code change, or could some trick work here?

Celluloid uses Pipewire natively and works without device access or Pulseaudio permission.

Yeah, this has always frustrated me on Android. It seems like a phone is the place where this functionality would be the most useful where you want different applications to be at different volumes.