I am a self-taught developer and currently I am working on a finance app and would like to ask you which way is more efficient to store my data in Firestore.

To say it in general, I have many users, but which also have many transactions and query them with a stream in real time. My question now is how it would be better to store this data and whether there are also advantages in terms of cost?

My structures to store this data would be like followed:

Which structure would you use, or is there even a better way to structure this data?

Thanks in advance for your answers