Firestore Rule to only give access to logged in users from collection

With this rule you will check not only if the user is logged in, you’ll also check that the logged in user’s UID match with what is stored inside a collection of users.

// Grants a user access to a node matching their user ID
service firebase.storage {
  match /databases/{database}/documents {
    match /users/{userId}/{documents=**} {
      allow read, write: if isOwner(userId);
    }
  }

  function isOwner(userId) {
    return request.auth.uid == userId;
  }
}