ITT we post mobile device apps that we like, but don't think everyone else knows about.
NewPipe is an unaffiliated YouTube app. It let's you play the video in the background, pop out the video so you can move it around the screen, stream videos like normal, or download the videos (or just the MP3 if that's what you prefer).
It's been a really nice app for those times where you just want to listen, so you can turn the screen off and enjoy the sounds. https://newpipe.schabi.org/
Not sure if it's in the Play Store or the Apple Store, but it's available on F-Droid. https://f-droid.org/packages/org.schabi.newpipe/