One of the features of Apple’s ecosystem is that everything syncs with each other. This means that photos, contacts, and notes are shared across devices that share the same Apple ID. However, if you plan to upgrade to the iOS 15 or macOS 12 beta, you might want to hold off if you don’t want to lose access to some notes.
This is because the Notes app will be gaining some new features in iOS 15 and macOS 12. By upgrading to the beta of either software, this means that if you were to create a new set of Notes on your device, a device running an older iOS or macOS build might not be able to access it due to compatibility issues.
For example, if you upgrade to iOS 15 beta but your Mac is still running on macOS 11, then the notes you create on your iPhone might not be visible on your Mac. You will also be greeted by an alert/error message that says that the note has been hidden on some devices. This isn’t that big of a deal, but it might be something to think about if you rely on the syncing of your Notes app quite a bit.
Both iOS 15 and macOS 12 are expected to be released later this year, possibly during the fall, so maybe waiting for the stable release might be a good idea.