How to use the same app bundle & based on minimum SDK & target SDK versions for the old API version it would be a very lite version of the app with less Gradle dependencies & code.
The old way to implement this to have two projects with one key store signing & to upload multiple Apks.
Is there a difference while using the app bundle or I have to generate multiple app bundles & upload them on google play?
Please note the core library is not applicable here because the full project is up & running.
You have basically two options:
Use of conditional delivery which supports serving parts of your app above or below a certain API version.
Publish two separate Android App Bundles with a different minSdkVersion and different versionCodes (similarly to how you published multiple APKs).
Hope that helps.