Android Architecture navigation Component with Bottom Navigation?
Having separate backstacks or not is a decision based on user experience but if you do want multiple backstacks for bottomnavigation then follow the link for correct approach by androidx developers.https://github.com/googlesamples/android-architecture-components/tree/master/NavigationAdvancedSample
I think it depends with the number of nested fragments you will have per each navigation item. The fewer the fragments then i would use the first approach and the more the fragments then i would use the latter approach