How to implement authorization on your own OAuth 2.0 server using social network accounts?

Question

OAuth

There is a Website with a database of Users who can log in through:
1. Native authentication (login/password).
2. Social networks and other third-party services (for definiteness, let there be VK and FB).
It is necessary to implement an authorization service (hereinafter referred to as AS) using OAuth 2.0, through which the Site mobile application (hereinafter referred to as the Application) will gain access to the Site API (including methods that operate on the User's personal information).
With the first group of users - everything is clear: the "User Credentials" grant (The application accesses AS, passing client_id, username and password).
Tell me, what grant and how to use it correctly in the second step?