system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: The process for remastering old games (PS1 etc) for new consoles (PS4/XBoxOne)
gpt
The first and primary thing done is porting the game. For these consoles that usually means there are different graphic APIs, and you need to update the game engine to use the newer APIs. Going from something like PS3 to PS4 shouldn't be a lot of work, Sony wants to make it easy for you. But stuff like PS4 to XBoxOne isn't necessarily easy, PS4 uses GNM and XBoxOne uses DirectX, there are other system APIs that change and everything in your game using them needs to be changed to match. Then you need to account for differences in CPUs and how your code interacts with it. On older/slower CPUs it was very common to use non-portable cpu-specific code to speed stuff up, it's less common with newer systems. Anyways, all that stuff needs to be updated to work on the new processor.

After doing all that, the game should mostly run as is on the newer system. Re-mastering additionally includes things like updating textures because the faster system can handle it and making a few improvement to the graphics engine. That is all normal game development stuff, and they do it as much as they want really.