Android 4.1.2 (Jelly Bean) is now considered a "Legacy" OS. The modern YouTube app requires much higher API levels. To get it working, you need to look for or older. How to Download and Install:
If the native app fails to load videos, try these alternatives: youtube for android 4.1 2 uptodown
It isn't perfect. It isn't fast. But it works. And for vintage Android enthusiasts, that is a victory. Android 4
Simpler approach: Use or Invidious API – they return playable URLs. you must follow these steps:
Because you aren't using the Play Store, you must follow these steps: