Where is the appropriate place to integrate Firebase Auth and Firebase Firestore user information?

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The appropriate place to integrate Firebase Auth and Firebase Firestore user information is in the backend of your application. This can be done by setting up cloud functions that listen for user authentication events and create or update user documents in Firestore with the necessary user information. Alternatively, you can integrate Auth and Firestore in your frontend application code by handling authentication events and updating user documents directly in your client-side code, but this can be less secure and less efficient.