Android had a concept of Intents by which you can launch the system apps using your own apps. Does Ubuntu touch have any options like that? If so how can one do it?
From simple QML apps, you can't launch an external application (the Ubuntu SDK will probably support this feature in the future though).
To start a system application, your Touch application must be using some C++ code an then rely on QProcess.
Regarding communication between apps, again using C++ you could use QDBus.