I have included features to login via Phone, Email-password, Google and Facebook in my mobile app. I know the phone charges after 10k. But my question is, does firebase always charge when it exceeds 50k users? Let's say there are 1 million users and the remaining 950k active users will be charged? Or is the Identity Platform something else and these costs only apply to the Identity Platform? Thanks in advance.
Firebase Authentication with Identity Platform is an optional upgrade of the basic Firebase Authentication product, and adds things like multi-factor authentication, blocking functions, and a service-level agreement to the product. If your project is on the Identity Platform upgrade and on the Blaze/paid plan, you will indeed be charged for all monthly active users above 50,000 - regardless of whether you use the additional features.
So if you're using any of those features, you are on Identity Platform. If you're not using these features, the easiest way to see whether your project is on Identity Platform at the top of the Authentication settings page in the Firebase console: