I heard people like playing Mozart to their babies, or was it Beethoven? If anyone sees this, please do tell me if the audio volume is too soft or too loud all of a sudden! This was subtitled on my phone instead of my computer so the volume might have changed!
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