Consult Android Studio Overview
Each instance of Android Studio contains a project with one or more application modules. Each folder of the application module contains the complete source sets for this module, including the src/main/and directories src/androidTest/. For the most part, you will need to modify the files in each directory src/main/for source code updates, the gradle.build file for the assembly specification, and the files in the directory src/androidTest/to create a test case.