Sample C /C++ code for "SUBDIRS" ?

[AnneRanch]

https://wiki.qt.io/SUBDIRS_-_handling_dependencies

After RTFM above I have some understanding how make/ qmake manipulates the individual projects. Now I need to put the projects together in code and I am kind-of stuck. I feel the best way is to have "lead dog " main project (window) with calls to subprojects .
In C terminology - include / define a function so the main can work with it.
How do I implement similar scheme in SUBDIRS ?

In very rudimentary words:

include sub-project
....
include sub-project

(run) main project

I could just add sub-project files to main project but that would defeat the whole SUBDIRS concept.

Any hint will be appreciated.

Any opposing views ?