How can I backup my Icloud synced notes locally?
The locally stored notes are stored in a database at ~/Library/Group Containers/group.com.apple.notes/
while iCloud notes are stored here: ~/Library/Containers/com.apple.Notes/
How to access the directories are described in this OXDaily link. https://osxdaily.com/2020/01/15/where-notes-stored-locally-mac/
There is a free app called Exporter, available from the AppStore, which will extract each note and save it as a markdown file.  MacDown and other apps will display the notes on the screen.
There is also a way to export all your Notes as pdfs using a script. I don't know much about this but found it on the internet.  The script and a description are given at https://macmost.com/export-all-of-the-notes-on-your-mac-using-a-script.html