If you have a note which you want to refer frequently, you can add it to the Quick Access. The note will appear in the sidebar and you can find it easily no matter which notebook or note list you are in.
Click the button in the top bar of the note editor to add (or remove) your note to the Quick Access.
Or use the shortcut Cmd/Ctrl + Shift + S to toggle the action.
Swipe your note to the right and select the star button to add/remove your note to Quick Access.
Or, tap the ... button in the upper right corner of the note editor and select Quick Access in dropdown menu.