I countdown from 100 while trying to count on my hands as well. I dont know why I can never get these countdown videos right! (aka sorry about the breathing and my nose getting in the way! haha)
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