I noticed that in the GCP free tier documentation there is a line that says
Free Tier resources are calculated per billing account.
The answer to the same question for Firebase isn't specified too explicitly in the Firebase Spark/Blaze plan documentation - but from my research, it seems as though it's calculated via project.
In the image: "All projects in project one share the same free-usage quota. All projects in project two share the same free-usage quota" (both projects share the same billing account in the picture)
https://firebase.google.com/docs/projects/billing/firebase-pricing-plans
It seems then that Firebase free-usage quotas are calculated per project while GCP usage is calculated per billing account.
If this is true, how does it work for example with, for example, Firestore? If I have two projects that are both Firebase and GCP on one billing account, GCP says I have 50,000 reads per month per billing account. Firebase says I have 50,000 reads per month per project. If each project does 50,000 reads, GCP (per billing account - 100,000 total) would charge me for the extra 50,000. But according to Firebase, it would fall under the free tier for each project (50,000 free each).
So what exactly determines whether to process free usage through GCP or Firebase? Is it determined via whether I started the project through Firebase or through Google Cloud? Or enabled Firestore through different pathways? How does it know which resource to associate with GCP (billing level) or Firebase (project level) to determine free-usage tier?