The field firestore gives you an option to have a separate Firestore database from your production Firestore database. This separate Firestore is an emulated Firestore database that lives in your cloud environment for the duration of your session.
The firestore field expects an array of collections. You have two options on how to fill an emulated Firestore database:
You can combine both the direct approach and getFromProd approach
To create a nested collection specify a full collection's path: