Google Play allows you to upload your app as an "app bundle" (aka: an *.aab file) instead of an APK. How this works is that Google will generate multiple APKs from the app bundle, split by architecture and image density, and digitally sign each APK with a keystore you've uploaded for you. This is a similar feature to what Apple provides on iOS and allows for smaller apps to be downloaded to the Android device.
You cannot install/run an app bundle on an Android devices. That is, Android devices still only support APKs. An app bundle is only a Google Play publishing feature.
This is why a Titanium "test" or "development" build should not build an app bundle, because it's useless for local testing purposes. It only makes sense for "production" builds for publishing.
When doing a "production" build, Titanium should generate both an APK and app-bundle to the destination directory. No configuration required.
Note that building a "production" APK is still useful. Some features such as push notifications, in-app billing, Google LVL licensing, etc. can't be tested with a "debug.keystore" signed APK. Also, app bundles do not support Google expansion files. Nor are app bundles supported on other Android app stores such as Amazon (ie: only supported on Google Play).
We can implement app bundle support once we've adopted gradle build support.
The gradlew command line would be something like this...