Getting .mp4 Files with yt-dlp
yt-dlp is an excellent tool for pulling video files off YouTube, however its default file output is .webm.
The following command will try to pull a native .mp4 file off YouTube and will do necessary transcoding if that fails after downloading.
yt-dlp -S res,ext:mp4:m4a --recode mp4 https://www.youtube.com/watch?v=dQw4w9WgXcQ