The most comprehensive resource if you want to learn more about Firebase in general, and how to integrate Firebase with the technology of your choice in particular, is the official Firebase documentation, available at the following address: https://firebase.google.com/docs/guides.

Still in the official documentation, you'll also find guides on the Firestore database at https://firebase.google.com/docs/firestore and Firestore Authentication at https://firebase.google.com/docs/firestore/security/get-started, as well as a specific guide on how to install Firebase with Flutter: https://firebase.google.com/docs/flutter/setup. For an updated list of available authentication providers, have a look at https://firebase.google.com/docs/reference/js/firebase.auth.AuthProvider.

For a detailed explanation about NoSQL database and for the different kinds on the NoSQL database, IBM has provided a very easy-to-read document at https://www.ibm.com/cloud/learn/nosql-databases.

A concept that may cause confusion the first time you deal with security is the difference between authentication and authorization. There's a brief but very clear explanation at https://auth0.com/docs/authorization/concepts/authz-and-authn.