Game-specific file-format ...

Here is the reference I use for file formats: wotsit.org. They have info on a few of the file formats you mentioned.

Most of the time these formats aren't publicly documented. They're probably only just barely documented internally.

The files that ship with the game have generally been processed from countless original assets, combined into an implementation-based format, possibly compressed, and otherwise made into something easy for the application to load quickly.

With a few exceptions, only games designed for mods release those file formats. The rest are just reverse engineered by people like yourself. Sites like wotsit are a clearinghouse for those reverse engineered formats. Generally a careful search on [google] will find it, if it exists.

The normal warnings about using other people's files are still in effect: Be careful that you aren't violating licenses, copyright law, or other legal elements.

- I owe the games.
-The only thing i may do is to explore and modify thoses files local at my
harddisk just for fun and never publish it.

Now tell which serious company cares about this ?