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

They contain the full key now which is why they are longer. Apparently it was necessary to fix the vulnerabilities in the previous version.

Public key obviously, not private.

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.

It doesn't cost anything to have a .onion. You just run Tor and enable hidden service mode.