[Dovecot] Dovecot and Postfix Authentication

I am a student and am trying to get a mail server setup for a project. I have all of the Postfix configuration taken care of, but I am confused by the way that authentication works with Dovecot in the mix. As a user of the system, am I supposed to authenticate myself to Postfix, Dovecot, both individually, or both together? My Postfix users are stored in a PostgreSQL database, should Dovecot also be connected to this db? Some basic pointers on how this setup ought to look is all that I'm hoping for.

Thank you, Christopher