Superseding the long used APK format (Android Application Package) is the AAB format (Android App Bundles), which was introduced back at the I/O 2018. This new Android app distribution/publishing method is a part of Android's modern development push. The AAB standard will soon become the default for all new applications published on the Play Store, starting August 2021.

Link: Android App Bundles to replace APK format and make apps lighter
via androidcommunity.com