How to modify base npcs in Gmod

How Do I edit the base npcs in Gmod such as Poison Zombie, Metropolice, Headcrab, etc
I know its written in C++ but where can I edit the files?
I looked in the vpk files but only models and stuff are their.
Where does gmod keep those files
and how could i edit them

It’s simple, you can’t edit them.

Seriously?!? please give me a real answer

That IS the “simple” answer, garry’s mod doesnt use default npcs, they basically “include” the npcs from the mounted game - eg hl2 etc.

That is the real answer, they are compiled into the game and you can’t edit the game. They are not written in Lua.

And no, contrary to the popular belief, no code or binaries from mountable games are mounted, that’s not how things work.