According to this random baby site: https://www.whattoexpect.com/pregnanc... “Your baby’s ears are extra sharp during these last few weeks. Studies show she may even recognize your voice and favorite songs after birth.”
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