Where does the app keep my data on my Mac?

Everything lives in two places on your own Mac, and nothing in this area is sent to anyone's servers. (1) The app's private data folder (its "Application Support" / userData folder) holds: a small SQLite database of your to-dos and lists (supernote-todo.db), an encrypted cache of your note PDFs (in a "notes" subfolder), sealed login/connection secrets (in a "credentials" subfolder), and a few small settings files. (2) Your saved note PDFs are written as ordinary, openable files in ~/Documents/Supernote. The keys that unlock the encrypted parts are stored in the macOS Keychain, not on disk in readable form.