Quick summary: I’m doing a room makeover, so I won’t be posting as much and the quality of my videos might be bad for a little. Also, I am not the one turning off the comments…have a great day!!
To listen while using other apps, switch the player to Picture-in-Picture (PiP) during playback — it keeps playing in a small floating window (the screen stays on).
To listen with the screen fully off, in-browser playback stops by YouTube's design. Open the video in the YouTube app to keep listening where background playback is supported (e.g. with YouTube Premium).
Open in YouTube app