What does it mean to add to existing contact?
Tap on Add to Existing Contact. You will see a list of your contacts. Scroll through the list or use the search field to find the name of your contact. Tap on the name. A contact card will open with the contact's information, including the new email address or phone number.
What does create new contact do?
3) Select Create New Contact. This takes you to a contact card for the person where you can add the same information as using the Contacts app above.