AES-256 Encryption key generation takes two inputs
User provided Password
SALT (unique for each installation)
Decryption can only be done by zettel notes app on user device. If you uninstall zettel notes app ( which clears SALT ) or forget password, notes can’t be decrypted.
Only asymmetric password hash is stored, which is compared with the user entered password on app start. Plain password is not stored in zettel notes.
OpenPGP
PGP Key encryption is provided via intergration with Openkeychain app.