How do you find functions and variables in the game to edit? Most of the time you'll just look through code for existing units and features. If it's not there, changes are you can't do it. Or can you?

No, likely you can't. But this is something to pass the time. These older games have minimal encryption and we're free to poke around. Here we use a hex editor to take a look through the game.dat file. Maybe we'll fidn soemthing, maybe we wont'. Learning the basic os reading hex is important to advanced modding and this is a very simple introduction.

For a more advanced hex tutorial, look up 'Command & Conquer' video #10 on CSF investigations.