After some experimentation during which My Contacts list was reduced from few hundred entries to Apple and myself I was able to verify ~/Library/Application Support/Address Book folder is where all of the address book entries are located in Catalina. I have also verified the ~/Library/Containers/com.apple.AddressBook folder is also involved as it gets updated when a change is mode to the Contacts List.

I also ran into a reason why replacing the ~/Library/Application Support/Address Book folder replacement may have failed in MG2009's case. When Contacts launches, if ~/Library/Application Support/Address Book folder is not present Contacts immediately creates the folder and structure with two entries Apple and the User. (Sound familiar?) When I restored the ~/Library/Application Support/Address Book folder I received no warning the folder was being overwritten and instead the restored folder ~/Library/Application Support/Address Book 2 was created. (I haven't figured out why I wasn't offered the overwrite option?) After a moment of panic when my Contacts list was not restored 😱 After discovering the renaming instead of overwriting, I corrected the folder names and violà the contacts list was restored. 😁

If we knew what it was we were doing, it wouldn't be called research, would it?

— Albert Einstein