I know this falls into the category of, "Now he tells me 🤬", but for the benefit of subsequent readers of this thread...

Your Contacts should be transferred automatically…
  • as long as both the old and new Machines are logged onto the same Apple ID and in System Preferences > Apple ID, Contacts is checked.
  • ...or you use Migration Assistant to migrate the network and user data
  • …or you restore /Users/username/Library/Containers/com.apple.AddressBook from a Time Machine backup
The entire Contacts list can be manually transferred by...
  1. Opening Contacts on your old Mac
  2. Select any entry
  3. Press ⌘A to select all the entries
  4. Click and drag to the desktop to create a .vcf file
  5. Open that vcf on the new machine which will launch Contacts and import all of the contacts from the old machine in a single operation.

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

— Albert Einstein