Comment by ignaloidas

Youtube has been long normalizing videos standard feed, switching to a -14 LUFS target in 2019. But LUFS is a global target, and is meant to allow higher peaks and troughs over the whole video, and the normalization does happen on a global level - if you exceed it by 3dB, then the whole video gets it's volume lowered by 3dB, no matter if the part is quiet or not.

The stable volume thing is meant to essentially level out all of the peaks and troughs, and IIRC it's actually computed server-side, I think yt-dlp can download stable volume streams if asked to.