Found this neat tip for watching a movie or TV show when you're doing an endurance ride in Zwift.
Only works with some tvOS apps that support picture-in-picture (only Apple TV+ app as far as I can tell? Doesn't seem to be supported in Plex, Tubi, or YouTube)
BUT if you play videos on those apps from your phone or tablet and AirPlay them to your Apple TV, you do get the PiP icon so that's a nice workaround.
If you want to know why some tvOS apps support PiP and some don't, here's more info