For Ark and 7 Days to Die, you can use this website to add them: click on Manage Server, go to the Players tab, and click Add Admin next to their name. Then restart the server, and they'll have admin privileges without having to type a password.
Ark and PixARK also have an admin password you can set under Advanced Settings. Anyone who knows the password can become an admin.
Using admin commands varies from game to game. You should Google for "[your game] admin commands" for more information.