Spotlight finds Notes via magnifying glass, but not via Finder "Find" or mdfind
The Notes app in macOS does not display notes as files on your computer regardless of whether they are synced to iCloud or not. 

The Spotlight service is designed to display relevant information to a user by having access to all Apple apps (and most App Store apps) and their content. Notes does not store its content as a simple .txt or .rtf files so it gets its information by querying the app for the content you provide it. The app responds and Spotlight will open the app for you. 

This database of content is rebuilt every time Spotlight indexes your computer. You can force Spotlight to re-index your machine by following the steps in this article: https://support.apple.com/en-gb/HT201716

Finder works differently as it will only search for physical files on your computer, meaning that notes will not show up.