What you want to know is "game porting". Porting is simple enough: take a code that is written for a particular architecture/OS/cpu/framework and make it to run on another. You can do this eitehr the short and easy way, or the long and hard way.
The short way is to emit the code for PC, PS2, XBox and other consoles simultaneously during the compilation process or to simply cross-complie. Small games that don't have much value/graphics/performance considerations can use this method. But games like gta and other big games prefer to fine-tune the game engine to take care of every performance features and critical bugs in each target platform.
Often, when you are porting your big game to a smaller platform or vice-versa, you need to rewrite the game engine from scratch to scale down (or up) the resolution, texture, models and other things.
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