When you merge a contact, the following steps are followed with the data
The winning contact keeps all of it's data
The winning contact gets extra data from the losing contact. So if the winning contact did not have a phone number, and the losing contact has a phone number, the winning contact will have the phone number added
All groups from the losing contact get added to the winning contact
All notes, tasks, campaign activity, property relationships, automations, invoices from the losing contact is added to the winning contact
Trust transactions will still have the old contact's names attached to them
The owner of the winning contact will stay the same
The owner of the losing contact, and any users able to see it, will get view permission of the winning contact
The losing contact is then deleted