I agree here that the comparison is between apples and oranges, but it has at least a near-similar round shape.
Cosmos DB: Account -> Database -> Container -> Item/Document
- Documents are grouped logically by a partition key
- Containers hold documents/items.
- Application-level hierarchy is usually modeled inside documents or through document relationships, rather than by nesting containers under documents.
Firestore: Database -> Collection -> Document -> Subcollection -> Document -> …
- Documents store fields as key-value data.
- Firestore does not use the same partition-key-based grouping model that Cosmos DB exposes.
- A document does not contain a collection as a normal field/object. It can have subcollections under it.
- Collections contain documents only
- Firestore is naturally hierarchical through subcollections.