How to Mod Games (.Gpd)

Hi, I am new to this community and I always wondered how people modded games such as call of duty 5 with usb (gamesave). So, few years back when I was playing on xbox 360, I really got interested in the subjet. Tried to find articles, but there was only tools that someone else did that would do the job for you! So, how do I do? Is it written in c/c++? Do I need to edit the gamesave using a hex editor? Thanks