One thing I’d like to see from an app like this is “force the video into an arbitrary file size limit” with a list of priorities to do so defined by the user. Say I’ve got a video I want to send over Discord (10mb limit) but I’m not intending for the vid to be fullscreened by the recipient so scaling it down to like 480x480 would be fine.

yt-dlp, but most of the time you’ll use some form of front end. All the “youtube downloaders”, even the shady websites, are using yt-dlp. For selfhosted purposes, I like MeTube for individual downloads and Tubesync for keeping up with channels.