The file rrbot.gazebo(introduced in ex2.2), how is it launched? Also, can we include the content of the files in a URDF file? By launch, what I mean is how does Gazebo know and run this file.
Yes, the .gazebo files have to be included into URDF files. In fact, all the contents of the .gazebo file could be placed directly into the URDF file itself. We just separate them in order to make the code clearer. So, they are loaded when you load the URDF files in your launch file.