I am using Firebase auth and Firestore for my app, and basically used the same flow as described/used in this repo, but I have 2 types of users and will have 2 types of UI depending on the user role. Default role will be a student, but in the event they upgrade to a Stripe subscription, they assume the role of coach and can take on students of their own. I have found that claims are the best way to implement this, but aside from that knowledge, actually using them properly is a concept that still escapes me.