yt dlp is what you are looking for

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

I dunno, mate. I dunno