Integrate phone OTP feature to an existing Firebase project and also optimize React Native app with Firebase fetches

I am currently building a React Native app, which needs to have phone OTP feature for authentication. Currently, I have setup a Firebase project and it allows users to sign up using their email and identifying information. The ask is to have a page before that that asks the user to enter their phone number, sends a one-time code, and verifies before letting the user sign up using their email. I've arranged the UI for it, but need assistance weaving a phone OTP functionality to the Firebase project I have. In conjunction to this, I need assistance optimizing my React Native app to avoid fetching data from Firebase repeatedly - thus incurring cost. It currently refreshes from Firebase every time I navigate to a new view and return back. I need the app to fetch the data only once and use local storage to retain information. Access to the GitHub project will be provided and I will prefer a one-on-one session to collaboratively work on this.