How do you make a backdoor?

There are multiple ways to go about backdoors, if you're setting up for a server, you can set up a MySql backdoor that allows you to gain access to their database and if that is connected to their in-game system you can gain access through there.

The other way is simple concommand statements that set you specifically to admin, these are easily catchable by someone who knows code but, if you hide it correctly it can work