There's a modified Youtube app thats better then the orignal app in every possible way, i want you guys to check it out. and im sure you'll switch to this app as soon as you watch what it can do.
Here's a video about its features and how its different then the orignal one.
Get it from here:
https://forum.xda-developers.com/android/apps-games/app-iytbp-injected-youtube-background-t3560900