Contact represents the end user on given channel. Contact is automatically created whenever you receive a message from the user for the first time. However, there are also different ways how you can create the contact (i.e. for Mobile platform the user is created when you send a message to the given phone number).
Contact Merging
Amio doesn't implement any type of contact merging for one user on different platforms.
See channel specific requests to see differences between contacts on each channel: