It did start with just youtube, yes. But in today's day and age it supports a large amount of sites. [1]

[1]: https://ytdl-org.github.io/youtube-dl/supportedsites.html

I believe it was originally just youtube, but it has support for a whole lot of services now including reddit, twitter, and much more.

Looking at the file names here should give you a pretty good idea of what is supported. https://github.com/ytdl-org/youtube-dl/tree/master/youtube_d...

Yes.

This is precisely what I am complaining about when I said that the SaveVideo spam means every other service that does not spam in the same way gets relegated to obscurity. Practically every downloader program/extension/service works with these videos...