Creating nested makefile

I am learning makefiles and I know how to create a simple makefile. I am moving on to nested makefiles. Here is my directory structure

/src
...makefile
...main.cpp
...foo
......makefile
......foo.cpp
......foo.h

When root makefile is called, it calls the makefile in directory foo. Here are my questions

1. Which makefile should I use to write code for linking all object files? If it is in the root makefile, do I need to specify all object file names there?
2. Is this nested makefiles a best practice? Or is it good to have only one makefile which is at root?

Any help would be great!