Apps provide multiple entry points
Android apps are built as a combination of components that can be invoked individually. For example, an activity is a type of app component that provides a user interface (UI).
The “main” activity starts when the user taps your app’s icon. You can also direct the user to an activity from elsewhere, such as from a notification or even from a different app.
Other components, such as broadcast receivers and services, allow your app to perform background tasks without a UI.