This was requested by SuperGamer87 some time ago. I just got approved for videos longer than 15 minutes so I thought it would be the perfect time to post this. This is mostly just a sounds video - nothing to see after the first few minutes.
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