While I’m VERY aware how illegal is is to steal game programs, there are so many cool ones so I have two questions, can all these cool mods for other games be stitched together, and is it legal to do so so long as it’s just for me and I NEVER share it.

Moving to "For Beginners".  Please use a more descriptive title next time.

48 minutes ago, Gold_fox said:

can all these cool mods for other games be stitched together, and is it legal to do so

No and no.

A game is a complex system of software.  Mods are extensions built to interact with very specific software.  As such, complex pieces from vastly different systems cannot be stitched together.  It is more cost effective to engineer a system that incorporates the pieces you would like to use than it is to reverse engineer and stitch together existing components.  There are just too many unknowns and complications with reverse engineering not to mention it is very likely illegal.

Assets and IP are protected by law.  When a question such as this comes up, I generally quote from VideoGameAttorney’s guide from a year ago, https://old.reddit.com/r/gamedev/comments/5pxldd/ultimate_as_promised_guide_to_legal_needs_and/:

Quote

SOME ANSWERS TO VERY COMMON QUESTIONS

• No, you can't make a damn fan game. Yes, it's infringing.  [...]  I've seen so many developer lives ruined (lost home, wife, kids, etc) all because of a silly fan game. These companies are brutal about protecting their IP. The reason you never hear about it? All settlements come with an NDA that makes it so no one can write or talk about it.

2 hours ago, Gold_fox said:

and is it legal to do so so long as it’s just for me and I NEVER share it.

Actually this one depends where you live. Some countries allow you to copy things like software, as long as you never share it or distribute it. There are even countries that have no copyright laws whatsoever. Some countries also have special rights, that allows people to break into any software legally.

So check the copyright laws of where you are.

2 hours ago, Gold_fox said:

can all these cool mods for other games be stitched together

Sadly it would be near impossible to stitch random software together in a working form.

You could clone them and making your own using them as a inspiration. No one owns an idea and that is a good thing or our selection of games would have been very limited.