Afaik there is currently no standard way to a) document the structure and organization of Firestore collections and documents stored in a project expected by apps which use it, and b) share the model definition across languages and platforms. Instead, each client requires its own implementation of the app-specific model and conversion to/from Firestore's model which needs to be kept up-to-date as the system involves.

If there were a way to share this model (for ex. proto, JSON schema, other) and easily use it from Firestore APIs, it would reduce dev and maintenance cost and improve developer experience.