Documentation: https://support.torproject.org/about-tor/onion-services/what...

There are also many web sites that provide an onion address in addition to their clearnet address. For example, the BBC: https://www.bbc.com/news/technology-50150981

If I ever get a .onion (everyone should have an onion probably) I'll also register the same domain "dot net, it's dot com" just for the lols.

And onion urls are a sha hash of i think the private key of the site