>>82691587yt-dlp is a program to download videos from Youtube. It merely downloads them.
ffmpeg is a program that allows you to convert media files into other media files, edit them, cut them, trim them, change framerate, etc.
So typically you download with yt-dlp, and then you edit them with ffmpeg. Maybe they come together as yt-dlp allows you to download something, and immediately convert them to something else, if you have ffmpeg installed. So yes, install both.
