Backend: The hosting is a database, people provide servers, host content, filter what they don't want on their own servers but if it's hosted by someone else on another server then it's available to users. In the end it works the same way as hosting any website except that you're not dealing with AWS or another such service, it's just people like you and me providing space on their servers to host chunks of the database and you back up everything so no one can wipe their server and make part of the database disappear

Frontend: The database is 100% public, if you create a website to access it all you're doing is providing the UI for users to see what's in the database and interact with it, you don't host the content itself

If you've ever played with crypto the principle is similar, the ledger is public, anyone can create a website to let people see the transactions on it and to push transactions to it