They're stored in an sqlite file at ~/Library/Containers/com.apple.Notes/Data/Library/Notes/NotesV7.storedata (version number might be different.)
The data stored all over the place in the DB.
For example, you can extract HTML body of a note with the following query: "select ZHTMLSTRING from ZNOTEBODY".
Yep. They probably use Core Data (which is a layer on top of SQLite) internally instead of straight SQLite, but either way it ends up on disk as an SQLite file. A lot of standard Apple apps do this.