1) The app will store your data to firestore under our company account. (Firestore is took care under Google inc's security department)
2) Your important data will be encrypted with your private key. That means if someone get the raw database from firestore, they still unable to read or understand your data.
3) According to number 2, if you lost your device and no back up in another device and even you can remember the private key, there is no solution for enabling to re-store your data. (But also for backing up data, you can install the app with the multiple devices at the same time)