Important Links
Firebase Pricing
https://firebase.google.com/pricing
Cloud Firestore vs Realtime-Database
https://firebase.google.com/docs/database/rtdb-vs-firestore
Cloud Firestore: Usage Quota & Limitation
https://firebase.google.com/docs/firestore/quotasRealtime-Database: Usage Limitation
https://firebase.google.com/docs/database/usage/limits
Realtime-Database: Sharding (Scaling with Multiple Databases)
https://firebase.google.com/docs/database/usage/sharding