Manipulating values scanned in memory. Basically, take an action in game and look for changes using software that scans for changes like Cheat Engine. When you find the values and address, then you can save them and modify them.
Anti cheats like EAC are supposed to block this kind of thing. However, not every cheat is going to be caught.
I do some programming projects from time to time, and modding some old games to support higher resolutions or to use more memory and such can only be done by scanning values in memory or using a hex editor to change the binary files.