The format of the Apple Notes data is undocumented and liable to change with macOS updates. It is not safe to rely on copying a single file elsewhere particularly where this contains Apple Notes's iCloud data.
If you want to continue using Apple Notes there is little choice but to rely on Apple's iCloud and Time Machine to correctly store and backup your notes.
But if you want to take control of your notes data I suggest you use a different app. This is what I have done because I did want to take control of my notes.
I have exported all my notes out of Apple Notes to markdown files using Exporter (also jmh's recommendation) and now use Typora to edit my notes knowing that each note is now a single text file and, in some cases, images.
So that I can access my notes on my other Mac, I store all the markdown format notes in my iCloud Drive. But not entirely trusting iCloud, I also copy all the notes to a local folder for which I can control backups.