How to build a project with more than one makefile, each in its own folder?

Question

GNU Make

Let's say we have a project.
Each logical part is divided into folders. Each folder has its own makefile, including the root one.
For example:

Project/
         Test/
              Makefile_test
              <...files...>
         Test2/
               Makefile_test2
               <...files...>
         Makefile_prj

How to launch from the side? What do you need to add to the "root" Makefile for this?

Answer 1

Ha, he asked, he answered.
Makefile_prj:

all:
        make make_test
        make make_test2

make_test:
        make -C Test/ -f Makefile_test

make_test2:
        make -C Test2/ -f Makefile_test2