Just letting y'all know about a pretty bad idea that YouTube has come up with. Good for some but not for us ASMR addicts. I will help you figure it out on your hand held device. Applicable only to Premium users.
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