How is an OpenID Authenticated user Identified?

On a system that I'm working on a user signs up by means of an OpenID/Social Media account (Twitter, Google, Facebook), and then subsequently signs in using that path. The System comprises of a Back-end (API and database) and several clients (Web Portals, Native Mobile applications, and User-Agent/JavaScript based clients.

These clients will get the Authorization to the Above-mentioned back-end API for a user from a special oAuth2 server. So to share registration data between the clients this will need stored in a central database, part of the back-end service. Basically I need to track the users in a database.

Once I have an Access Token from an IdP for a user I can get some details about that user, eg Name and Email address, and I'd store some of that in a local database. I've previously asked "what I need to store" and didn't get any satisfactory answers, but now I've got a more specific question: What will the IdP give me to identify the user? Some kind of unique URI for that user? An Email Address? An IdP specific ID string?

I need to figure out how I go from getting an Authentication done against the IdP to getting an Access code from the oAuth service for the back-end. Once I know what I will have available from the user's IdP I can go to the next step, which is to figure out this part between Authenticating the user and getting an access token from the Authorization server for the API.