Link to a note in Apple Notes, from another note
Adding an additional method to Seamus'excellent answer from above… this one keeps you entirely in the Mac app:


In the Mac app, go to the note that you want to reference ("Donuts I've Eaten") and click the Add people to this note button in the Notes toolbar.



  



In the dialog box that pops up, choose Copy Link and then Share. The Share button may be disabled as soon as you click Copy Link. You must then enter at least one contact (an email address or a phone number)  in the Add: field. Note that the contact must be different from the current iCloud user on your Mac, otherwise Share will be again disabled. This step also makes the piece of note permanently sharable until you remove the share. You can always come back to the note and copy link from what's behind the Add people to this note button.



  



The note's unique URL is now copied to your clipboard. It should look something like this "https://www.icloud.com/notes/0-QF4Bf9jmhfersTKH16_2_kA#Donus_Ive_Eaten"
You can paste that URL into the note that you want to have the reference in. But, even better, highlight the words that you want to become the clickable text, type Command+k to bring up the "Link Destination" box and paste it in there. 




In this method, the note is technically shared now. But since it's not shared with anyone else, it will just serve as a quick link/reference for your other notes. It's a bit of a hassle, but it seems to work.



I should also mention that the first time I tried this, I think I was forced to add an email address in the "Add:" box before I could click Share… but in subsequent trials, this was unnecessary. So I'm not sure if it was a first time only kind of thing, or if I just goofed.