What is the difference between an APK and an AAB?
An APK (Android Package) is the installable file that runs on a device; an AAB (Android App Bundle, .aab) is a publishing format you upload to Google Play, which then generates and signs optimised APKs for each device. Since August 2021, new apps must be published as AABs — you can no longer upload a plain APK for a new app. APKs are still used for sideloading and direct distribution.