How come people have @username@ditto.pub addresses?
Ditto is awesome and does Nip05 Addresses too. Everything all rolled into one
No app? Only on web?
Correct. You can probably use it as a PWA on phone. I do that with my fork of nostrudel.
Oh, wait, I get it!! So Ditto by Soapbox is basically a web UI and you can just hook it up to your own. relay to make a fully functional community ditto.pub is just one such community. There can be as many as you want. Each will have its own NIP-05
No, ditto is a relay that also installs the soapbox web ui with it