It is very rare that your app will not make use of some other apps in your phone or some of the system apps present on your device. Hence, testing of such cases where your app has to open another app, make use of that app, and come back, involves some more serious testing. These test cases have to verify that the interactions happening across the other apps are as expected.
When your app has interactions with other apps, it has to first call the other app from your app, open the other app, and see if that app displays the expected behavior, that your app wants it to do, and finally get the response from that other app, if required. If there is a response that is returned from the other app, then it has to be verified and checked if it is as expected by your app.
There could be two examples that I would give. The first in which your app does not require...