There can be multiple module build.gradle files in an Android application project. These files are located in their (module) directories and are visible in Android Studio in the Android project view inside the Gradle Scripts section. Most customers are using the module name "app" for their application modules, because it is the value of the default name from the Android project template. Some customer have changed the module name and then the application module can be determined via the plugin id:
apply plugin: 'com.android.application'
The application module build.gradle file contains the most important configuration options for the Android app (link minSdkVersion, ...). It also contains build-relevant settings like build/packaging options, additional build types and product flavors, variants etc.
When opening the project in Android Studio, in the Android view (Left pane), there is a Gradle Scripts section. Inside, there is a file called build.gradle (Module: app):
