This is a small tutorial, so simple it can seem obvious once you understand the process. it’s useful as a next step after the design of a sum of shapes. Once done is uncomfortable manage all the pieces and all the masks added.
In this example you can see I created a fake space with the gray rectangle over the blue rectangle.
Select the three shape and open the Pathfinder Window (Shift+Ctl+F9). Click Merge.
The path is now all joint; with the white pointer (Direct Selection Tool) select the gray shape and delete it (Canc).
Voilà! the path is a one piece and follows the shape you want!
Screenshot: