I want to migrate my android app (~40 MB of the base apk) that uses an .obb file (~70 MB) to the Android App Bundle format. I know for sure that the resources inside the .obb file will be updated less frequently than the base part. Therefore, there is no need for the users to download a full app (40 + 70 = 110 MB) on each app update but only the changed part (~40 MB).
What can I use after the migration to provide the same experience (update only changed part) to the end users?
Ideally, these unchanged resources should be accessible through the AssetManager like they are located within the assets directory.
For now, I found the Play Asset Delivery feature that allows moving some resources to a separate module to deliver it to devices at different times (install time / right after the download / when needed). I've implemented an install time module logic and tested it via the internal + alpha tests. Throughout the testing, I found out each app update forces Google Play to download the full app (~110 MB) instead of the changed part (~40 MB). I do not know whether this situation will change when I release an app to the "available for everyone" track and I could not find the information about it either. Therefore, the answer "an install time module will not be redownloaded if unchanged; here is the documentation describing this: <link>" is fully acceptable and is one of the most welcoming ones.
tldr; Yes, in production, the download size will be smaller.
When you publish an app to production (whether APK or App Bundle), Play will generate patches of your app between this version and the previous versions of your app, so that your users only download the difference between the two versions. The consequence is that if you don't modify the content of a dynamic module, users will still receive updates for the unchanged modules but the download size will be very small.
Note that in practice, it is possible that the first couple of users downloading the update may still get the full size.