Subdirs is just a template that allows you to have several separate .pro files bundled together: you run qmake once for the root .pro file, and the rest is built automatically.
For a huge example, simply check out Qt code: all modules are based on qmake SUBDIRS. For a smaller example, take a look at my "QEasyShell":https://github.com/sierdzio/qeasyshell.
If you want other projects be rebuilt when you update a dependency, you need to use DEPENDPATH variable.