What is Firestore?
Firestore is one of many products in the Firebase product line. Firestore is a document-based NoSQL database. Firebase Auth integrates with Firestore to provide authentication/authorization functionality.
The benefit of using Firestore is that developers don’t have to deal with the hassle of managing servers, creating a backend, or scalability. All of that is handled by Firebase. As such, Firestore is often used by frontend developers.
Firestore is fundamentally flawed. By eliminating the logical layer, Firestore makes it nearly impossible to use it securely. Firestore security rules are an inadequate band-aid solution and cloud functions defeat the purpose of using Firestore.
There are very few situations where Firestore will be an adequate solution. I’d only recommend using it only extremely small and simple applications or mock applications, but even so, you may run into security limitations.