I think this is a new feature that came with Big Sur, but now the system has a very convenient sub-menu on the main menu bar showing all current media sources, solving the problem of hitting the play/pause button and controlling the wrong source.
Example use case:
I'm listening to Spotify and I open YouTube for a quick tutorial. Video starts auto-play and I need to pause Spotify to listen to what the person is saying. I tap on the media pause button and I am now pausing the video, since it was last played source.
It'd be good to be able to customize the “Now Playing” widget to allow for showing multiple sources somehow, so that this could be avoided. It seems that system now offers this capability