ffmpeg can't do the actual downloading, but if you do any converting/merging with youtube-dl it will use ffmpeg to do that. However ffmpeg can stream/download from rtmp and similar, but not YouTube, it's mostly just for doing literally anything with video files (and sometimes images).

I downloaded and converted (to mp3) so many hour long mixes. My car is pretty old but it can play mp3 from an SD card, so long mixes are great for roadtrips

yt dlp is what you are looking for

I dunno, mate. I dunno

Nope, it can't. You'll have to continue using yt-dlp/youtube-dl for that. Both actually use ffmpeg to merge the downloaded audio and video streams into a single file.

I just want to point that you may have some issue with duplicated requests to your lemmy instance.

You should be able to process video through ffmpeg after downloaded from youtube.