connection between launch file and src file in moveit

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I was learning moveit tutorials, but since the moveit full package was not avialbe for pr2 in ros indigo, i can't understand how does the src folders of moveit(eg.move_group_interface.cpp) and launch file of moveit_configuration folder(eg.move_group_interface.launch) are connected and how do the src file is executed when the launch file is launch by using roslanch. And i am not that experienced with launch files, yaml files etc. But learning day by day.