Tor is a network of computers connected together USING the internet. Together they act to anonymize connections through the network through a process called Onion routing where sending a message involves adding multiple layers of encryption to it so that no computer in the chain can know both who sent it, and what it contained.
There are things “hosted on Tor” because it IS the internet still, you can access a Tor routing computer because it IS ON THE INTERNET, a Tor router can just host an internal website, and the masking of Tor helps hid where that website is.
Anyone can host whatever this way, just like anyone can host whatever the normal way, its just harder to locate.
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