Contacts - Link vs Merge
This has been asked in the past, but I couldn't find a good answer. I did some digging and found the following.
LINK Contacts:
... you may have multiple cards for a contact—for example, one card from your Yahoo account and one from your Google account. You can link the cards to streamline your list of contacts. The card for each account remains, but Contacts shows just one card with the combined information.
MERGE Contacts:
If you have more than one card for a contact in an account, you can merge the cards. Merging cards combines the cards and keeps all the unique information in a single card.
References:
https://support.apple.com/guide/contacts/merge-contact-cards-adrbk1456/mac
Hope this is helpful.
MacBook Pro 13″, macOS 10.12