When exporting from android select android app bundle not .apk https://developer.android.com/guide/app-bundle
Buildbox does not yet support .abb .abb It is a future format to create APK based on each phone where it will be installed. With .abb files, applications/games would take up less space.
I forgot to type android studio. Export from buildbox as android then import in android studio. In android studio when you are exporting(building) the project select build as .aab not .apk