Financially is a local ledger for iPhone and iPad. There is no account to sign in to, and nothing is sent to a Financially server.
Where is my data? On this device only, plus any backup file you export.
Do widgets see my full ledger? No. They read a summary through a private App Group on the same device.
What does App Lock protect? It asks iOS to authenticate you when opening the app. It is not encryption of the store.