Your recovery words, the optional passphrase and your PIN are kept encrypted on the phone, protected by the Keychain on iPhone and by the Keystore on Android, and are never sent to Voltz or to any server.
The Breez SDK Spark, the wallet engine inside the app, keeps the wallet's own data on the phone, such as its payment history, and keeps it in step through Breez's sync server.
Wallet names and photos, the contacts you save and your settings stay on the phone. Contacts also pass through Breez's sync server, encrypted with a key only your wallet holds. The camera only reads QR codes, and a photo you pick for a wallet is made smaller and kept on the phone.
Face ID and fingerprints are checked by the phone itself, and the app learns only whether the check passed.
On Android, the app keeps its data out of the phone's cloud backup. On iPhone, a backup of the phone can include the app's data, such as wallet names and payment history, but the recovery words and the PIN come back only on the same phone.
Removing a wallet in the app deletes its recovery words, its passphrase and its photo from the phone. Its name, its payment history and its contacts stay in the app's storage.