as i understand it
, google play generates an APK from the AAB by taking parts of the AAB relevant to the requesting device specs:
Quote: "What is the difference between AAB and APK files?
AAB files are custom-made
The difference between APK and AAB files lies in what we call resources. In an app, resources are everything else that is not programming code, such as images, language files, or music. If you select the "Install" button for an app in the Play Store, Google will then send you an APK file tailored from the AAB file for your device with the necessary resources required.
Your smartphone only has Full HD+ resolution? Then you'll receive an APK without 4K material. You've selected English and German as the device language? You probably won't need the Swahili language pack. By the way, with the help of Google's bundletool tool, AAB files can also be locally converted into APKs."
...with the (whole) point being smaller file size delivered/installed.
re: "AAB files can also be locally converted into APK"
...i expect you'd need an app on the device to do that for you. IE, the device doesn't know what to do with an AAB natively. if it did and we download an AAB to pick through, it defeats the whole point of AAB?