Android App Bundle documentation is coming alongside the release of 4.25. You should be able to check it out under 4.25 Preview 1. However, I will caution that they aren’t really used for splitting up your game assets. Rather, they’re an alternative to APK/OBB builds that’s proprietary to the Google Play Store, used for condensing the upload process for your projects. Where you previously would have needed to make separate APKs for different sets of devices based on texture formatting or other considerations, you upload one AAB file and Google Play uses it to create a final APK customized for the end-user’s device. It’s convenient, and the final APK that’s distributed to the end-user has a limit of 150 MB instead of 100 – but an app bundle is not to be confused with an asset bundle.
You’re looking for information about chunking, pak files, asset management, and patching, I think, which is one of the other topics I’m currently working on revising documentation for. Basically, you can download sets of .pak files and have a device mount them to make their assets accessible. We’ve got robust tools for packaging assets this way that are fairly intuitive to use (see our section on the Asset Manager and Primary Asset Labels), but the process of patching itself is definitely a bit hazy – mostly because different platforms each have their own ways of delivering these kinds of files. Nevertheless, I’ve invested a lot of time in researching this process for mobile, and you can look forward to some new/revised documentation for it soon. 