Earlier quoted context omitted.
Try NewPipe, it's an open source YouTube downloader and background player for Android: https://f-droid.org/repository/browse/?fdid=org.schabi.newpi...
I just tried NewPipe. However, after downloading a video, you can no longer play it directly in NewPipe (with NewPipe's ability to play in the background), only in an external player.
For that reason I call it NewPipe/VLC, or as I've recently taken to calling it, NewPipe plus VLC.