That’s correct, Cloud Firestore cares less about the amount of downloaded data and more about the number of performed operations.
As Cloud Firestore’s pricing depends on the number of reads, writes, and deletes that you perform, it means that if you had 100 users communicating within one chat room, each of the users would get an update once someone sends a message in that chat, therefore, increasing the number of read operations.
Since the number of read operations would be very much affected by the number of people in the same chatroom, Cloud Firestore suits best (price-wise) for a person-to-person chat app.
However, you could structure your app to have more chat rooms in order to decrease the volume of reads. Here you can see how to store different chat rooms, while the following link will guide you to the best practices on how to optimize your Cloud Firestore realtime updates.
Please keep in mind that Cloud Firestore itself does not have any rate limiting by default. However, Google Cloud Platform, has configurable billing alerts that apply to your entire project.
You can also limit the billing to $25/month by using the Flame plan, and if there is anything unclear in your bill, you can always contact Firebase support for help.
Regarding your second question, a read occurs any time a client gets data from a document. Remember, only the documents that are retrieved are counted - Cloud Firestore does searching through indexes, not the documents themselves.
By using subcollections, you can still retrieve data from a single document, which will count only as 1 read, or you can use a collection group query that will retrieve all the documents within the subcollection, counting into multiple reads depending on the amount of documents (in the example you put, it would be 10x10 = 100).
