How do I properly set up Postfix with a list of my domains that I want to use in it?

Question

FreeBSD

There is postfix, postgresql, freebsd - for hosting your own soap.
I'm setting up. Questions:
1) What is a relay domain in general? There is no clear definition anywhere. Everywhere only as it is configured.
2) How do I list **my domains** for which I want **this** Postfix to send and receive soaps? Namely, let me have domain1.com, domain2.com, domain3.com. Postfix itself is generally on a third-party server.
I want to be able to send and receive mail through them. Where exactly is this written in my Postfix? Or do these 3 domains need to be added to the Postgresql database first?