for what the differences are - check out both Repositories to somehow same versions before they diverge to much and rename classes / move folders, then do recursive folder diff.
For Nextcloud related content, what keeps Contacts (Aosp version), Calendar (Etar) and Tasks (opentasks) in sync is the work of davx5-ose (formerly Davdroid). So it’s really the protocols (Web-/Cal-/CardDAV) doing the interfacing.
What I wonder whether it’s worth maintaining a fork for most of these. Especially since its already seems some /e/-apps are missing features from its upstream (e.g. Sms app misses features from QKSMS).
I don’t know how else to create a more uniform UI though …so just throwing this in as a question. It may very well be worth it.
I think there is no other choice to forking if a handful of people try to quickly make their case and show their vision as it allows for more control. The /e/-Team made excellent picks in what to fork. But then you’re also on the hook for running the bugtrackers and having divided the userbase.
Longterm the user benefits if there’s close collaboration. If you fund both projects, it’s also a more efficient use of resources.