Do we need foundation for V13? Is yes, how could we install that in running instance?
Foundation is not used in V13.
However, if you understand Frappe already, you can use foundation (or any frontend framework) to develop your portal pages.
You can also develop frontend pages that communicate via api to Frappe / ERPNext.
The motto of Frappe is less code. Just install Frappe and look at the tutorials. No need to install other things like Foundation,