composer: What is the difference between rootpath and vendor

I am new to composer and there is this one thing I do not understand:

Installing CI via composer creates a folder called vendor. All the extensions and stuff get installed there. Up to here everything ok, but why is every folder of CI (but system) twice? Why is CI under vendor at all? Shouldn't it be enough with the rootpath? How does CI know where I changed what file?



If somebody know where to find the answer to this I would be happy to follow any link. 

Cheers

Hi, I have answer similar question here: https://forum.codeigniter.com/thread-757...t=composer

Thanks, I'll continue from there.

Cheers

It is easier to search the internet than posting a question. So, yes, I am asking because I did not find the answer anywhere else.