It probably depends on the game and how they store server information. For instance, if they hostname for the server that broadcasts available game servers is sitting in a .ini file, you don't need a crack. If it was a hardcoded string in the binary, then you would. Of course some games may also employ other mechanisms to restrict what/where you can connect to, but in a lot of cases I've seen it is a matter of pointing to an IP with a server running.

As an example, Counter Strike: Source stored the servers in a human readable file and you could add alternative servers in the config file without a problem.

And where do you get the software that runs on the servers?

Most server software is emulated but I suspect there might be instances of leaked server software.

In 2007, at least, you could play WoW on a private server merely by changing your config file to point at that server; no crack was necessary. It probably was not worth Blizzard's time to shut down this configurability.