Difference between Google Cloud "Cloud Identity", "Firebase Auth", "Identity Platform"

I am trying to understand the difference between: Cloud Identity, Firebase Auth, Identity Platform

I've read the following documents: https://cloud.google.com/identity-platform/docs/product-comparison https://cloud.google.com/blog/products/identity-security/identity-and-authentication-the-google-cloud-way

My questions are:

1. Why is identity platform part of the marketplace and not a generally available service?
2. Identify Platform seems to be a subset of Cloud Identity, why?
3. In what cases should I choose which service?
4. If I decide to use Identity Platform instead of Firebase Auth, would that affect the way firestore rules are done? Because Firestore rules are dependent on Firebase Auth
5. Can any of these auth systems be shared across projects. For example, I am building various business objectives into functionalities that are grouped according to projects. I want 1 auth mechanism to work across all these projects to secure API requests with JWT tokens, ensure Firestore rules still can be used effectively if each project has its own Firestore database, secure access for client SDKs to access database products like Bigtable or Bigquery and provide access to frontend apps.

If you need to know my use case, I am basically building a modular enterprise app that would be used by my company for our construction projects.

Thanks everyone!

If I decide to use Identity Platform instead of Firebase Auth, would that affect the way firestore rules are done?

When you're using Cloud Identity Platform you are still using Firebase SDKs to sign users in to your app, and their token information is still available in exactly the same format in your Firestore security rules.