Starting August 2021, Google Play will require all new apps to be submitted as Android App Bundles (AAB), which provide greater efficiency that APKs, says Google.
App bundles are signed binaries that organize your app’s code and resources into modules [...] Code and resources for each module are organized similarly to what you would find in an APK—and that makes sense because each of these modules may be generated as separate APKs. Google Play then uses the app bundle to generate the various APKs that are served to users, such as the base APK, feature APKs, configuration APKs, and (for devices that do not support split APKs) multi-APKs.
It is important to notice that AAB will only be used by developers to submit apps to the store, while APK will remain the format used to distribute apps to users' devices. Only, they will receive tailored and optimized APKs.
According to Google, the AAB format grants a number of advantages over APKs. First, the optimization enabled by AAB makes apps served to users 15% smaller. Additionally, AAB provide a way to customize what features are effectively delivered using Play feature delivery, and support dynamic delivery of large assets using Play asset delivery.
Play feature delivery provides four delivery modes: install-time, to deliver a feature when the app is installed; on-demand, which enable to request features as needed; conditional, which makes it possible to specify install requirements, such as minimum OS version, device capability, and others; and instant delivery, which makes it possible to preview the app without installing it.
Play asset delivery supports three delivery modes: install-time, on-demand, and fast-follow. The fast-follow mode will download asset packs automatically after the app is installed without waiting for the user to launch the app.
Another advantage provided by AABs is support for Google's secure key management service, which prevents credential from loss or compromise. This requires developers to handle Google their signing certificates, which are required to sign APKs before distributing them to users.
AAB is supported by most build tools, including Android Studio, Gradle, Bazel, Buck, and others, which will make it almost transparent for developers to adopt the new format.
Google introduced the Android App Bundle as a mechanism to publish apps on the Play Store in 2018. Since then, over 1 million apps adopted it among approximately 5 million apps overall. While this figure might not appear extremely large, Google maintains that the majority of the top 1,000 apps and games adopted the format.