They can. IIRC, Amazon apps would check to make sure the Amazon App Store was still installed. And I’m pretty sure Netflix games stop working when you unsubscribe from Netflix

Because the developers want to check whether you got the app from the Play Store.

If the developers don’t care where you get the app from, then they won’t check.

For one-off file transfers between any two devices, I like to use justbeamit.com

For a long term link, use syncthing

Maybe get a 3.5mm to USB dongle and just keep it attached to the headphones?

Then every streaming service will die except Disney.