About using iCloud Notes on Mac
You can store notes in iCloud so that you can view and edit them on all your devices, as well as iCloud.com. Notes stored in iCloud appear under iCloud in the sidebar of the Notes app. If you don’t see iCloud in the sidebar, you need to sign in to iCloud and turn on the Notes feature in iCloud preferences. See Set up iCloud features on Mac.
If you see an Upgrade button next to iCloud in the sidebar of the Notes app, you haven’t upgraded your notes yet. This means that until you upgrade, you can’t apply paragraph styles or checklists, use the Attachments Browser or the Recently Deleted folder (for temporary storage of deleted items), lock your notes, or share notes or folders.
If you upgrade your notes, you can view and edit them on other devices that use OS X 10.11 or later, iOS 9 or later, or iPadOS 13 or later, and that are signed in to iCloud using the same Apple ID. You can’t access upgraded notes on devices that use earlier versions of OS X or iOS. Also, any notes you create or edit on those devices won’t appear on devices with upgraded notes.
When you upgrade, most of the formatting in your notes stays the same. However, any notes with HTML formatting (notes you created by copying and pasting from a website) may look significantly different.
Another type of iCloud Note account is from a Managed Apple ID account, which you may have from your school or business.
See iCloud Support.