WHO USES UDP?

Everyone.

Name lookups on the Internet are mostly tried first over UDP, and only fall back to TCP if that isn’t going to work.

So every time you load a web page or an app does something on the network, there’s a few UDP packets to start the connection process.

Also, HTTP has traditionally been TCP-based… but HTTP/3 runs over QUIC, which is a vaguely TCP-like transport that runs over UDP.

Since all Google sites, Facebook, and many others are QUIC enabled, as are most browsers, you might well have been browsing over UDP a lot without knowing it.

Also, and a bit more obscure but still necessary, NTP is used to set clocks on basically everything on the Internet, including client devices. Since you can’t do certificate security without accurate clock settings, this really matters. And NTP uses UDP.