Extracting moddable files from a game Thread

This is a fairly basic modding question, and probably one that's been answered many times before: how do you extract workable files from a game's unmoddable main files?

I'm guessing the procedure's different for every game. But how? Are there certain ways of extracting that work with multiple games, or does every game require a different method?

Over the years I've seen many games that were broken open by the community. And I've never understood how they did it.

The game that prompted this question is Trine 2. I'd like to replace some UI graphics, for fun and learning. So how would I go about that?

Edited by: Qoiorav

I'm not sure if you can change Trines unmoddable files. In most cases that a games unmoddable files have been changed, its due to user-created decompilers/recompilers and generally these are only made if someone needs some tools the developers don't provide.

Trines tools were not released long ago so it may take time before tools like this appear.

Ah yes decompiling, that's the word I was looking for. What if I wanted to create a decompiler myself. How would I go about it? Would it be overly optimistic to assume I could create such a tool myself?