Are locked notes in the Notes.app are encrypted on the local host? Anyone has a clue? I know they are encrypted on the servers, but I would like to know if they are also particularly encrypted on my machine. Thanks!
All user data on iOS devices is automatically encrypted. This includes any data stored in the Notes app.

Data on Mac computers is not encrypted by default, unless the individual app encrypts its data storage. It appears that data is stored in an unencrypted SQLite database, based on the fact there's a third-party export tool available.