The Android App Bundle (.aab) is a new upload format that includes all of your app’s compiled code and resources, but defers APK generation and signing to Google Play at install time.
Google Play does this through a new app serving model called Dynamic Delivery, this uses your app bundle to generate and serve an optimized APK for each user’s unique device configuration, so they download only the code and resources that are needed to run the application.
This eliminates the need to have to build, sign, and manage multiple APKs based on their ABI, locale, screen size, and more. Additionally, you can use dynamic feature modules to define what feature modules and resources that users first download when they install your application.
Using Android App Bundles increases the app size limit to 150MB without having to use those pesky APK expansion files. One of the many benefits you get when using Android App Bundles.