There are a couple of reasons to use REST over the SDK:
Unfortunately even though a lot of the Firebase services can run without Google Play Services now; the Unity SDK still requires it.
The Firebase Unity SDK for Android requires Google Play Services, which must be up-to-date before the SDK can be used.
Rest Client for Unity: https://assetstore.unity.com/packages/tools/network/rest-client-for-unity-102501
Step 1 — Enable Authentication in Firebase
In your Firebase project go to Authentication and enable Email/Password in the sign-in methods. …