I'm pretty new to Tor and don't know much about it. I was wondering how one would take control of a Tor hidden service if they had the private key. There's a darknet website called "DOXBIN" that was seized by the FBI about a week ago; the owners then gave the private key to their friend who re-gained control of the site with it. How did he do this? I'm really curious. I couldn't find anything on Google. Thanks.
You'll just have to put the privatekey along with the hostname file in folder, and add the corresponding HiddenServiceDir and HiddenServicePort commands in your torrc file. Just like what you would do when hosting a new .onion address. When you do that, anyone trying to access the .onion site address in question will reach your site not the old one, since you've declared your ownership of the .onion domain later. The original can re-declare his ownership after that just like you did, so it can become a loop, but he cannot change the private key unless he wants to change the .onion domain.