Here are some common questions about what happens when a contact is merged:
What happens when contact records are merged?
All related objects are combined into a single contact
This may mean duplicated related objects for any custom objects (such as organization affiliations)
Whatever records were not kept as the “master” are sent to the recycle bin. If a record was sent to the recycle bin in error, it can be recovered for up to 15 days. However, if you undelete the record, it will not separate the merged related records.
What happens to user accounts if both contacts had them?
First, you should know that nothing in the lightning contact merge shows which has a user account. However, it does a better job of handling user accounts than classic. In classic, it was much easier to accidentally make a user account inactive without realizing it.
The Master record User remains active. If for any reason the contact email address and the user email address are merged from different records, they could be different, which is ok, but not ideal.
The contact records that are NOT the master and therefore are deleted, the user record will be made “inactive”
https://help.salesforce.com/articleView?id=contacts_merging_contacts_associated_with_portal_users.htm&type=5
What happens to Pardot if the merged contacts each have a Pardot record?
The next time Pardot syncs with those records, it will take action on the Pardot side, similar to what happened on the Salesforce side.
https://help.salesforce.com/articleView?id=pardot_sf_connector_merge_delete_setting.htm&type=5