Composer question

Hi, we are working on that behind the scene, discussing tutorials and such. Did you understand what the concept behind it after my description? So I know in what direction we need to take things.

Well, to be honest, I don't understand it. 
The system folder stays "uncopied" for some reason. 
Besides from that I wonder why I want to update a file in vendor but not the same file in my root?

Let me take the file controller/Home.php as an example. You add a new method there in a new version of ci4.  
I see the update and I run composer update. Does both file change or only the one in vendor/  ?
If only the one in vendor/ change, how do I update the one in my root/app/controller/ ?