How do I edit everything?

[Your Computer]

But how would I edit the actual scripts the game uses to run? Such as how a monster would find you. I'm pretty sure you can't edit that but whatever I would love to know

I had assumed you knew what a plain text file is. Have you ever seen a file with the extension .txt? Do you know how to edit .txt files? It's the same thing, though the extension of a file is practically irrelevant to its content.

If, however, you meant the actual core functions accessed by the binary executable of Amnesia, then you're going to need the source code of the HPL2 engine.