MP4 vs WebM
MP4 and WebM are containers, not quality settings. A 720p MP4 and a 720p WebM can look similar. They differ in which devices open them without extra software.
Choose MP4 when you need it to just play
iPhones, Android gallery apps, Windows, and most editors expect MP4 with H.264. If you are sending a file to someone else or editing it later, MP4 is usually the safer pick when YTSave lists it.
WebM is not a broken download
WebM often uses VP9 or AV1. Browsers play it well. Some desktop players and messaging apps do not. If YTSave only lists WebM, that is what the source provided. Converting it yourself is optional; the download is still a real video file.
What YTSave will not do
The site will not pretend every URL is MP4. If you need MP4 specifically, start with a source that offers it, or use the MP4 downloader page and pick an MP4 row when one appears.