Courier font isn’t available anymore. It was replaced by courier new. Why?
It’s available at hundreds of websites. Google search will get you there.
Also note: If I remember correctly Scrivener installs “Courier Prime” by default. 
It’s definitely a better Courier than “just” Courier (or “Courier New” for that matter). Give it a try.
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If you are writing a screenplay, do not use Courier New. It is not a proper 12-point, 10-pitch monofont, and will change the true page count of your screenplay.
Have you tried opening Font Book to see if Courier is still on your system? The substitution to Courier New may be a Scrivener issue, rather than a missing font.
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If Scrivener doesn’t see it, the font is not installed.
On the Mac, Scrivener doesn’t install any fonts of its own, it uses the Mac OS font set.
There’s an old and venerable saying about assumptions. (link NSFW) I prefer to verify that everything is working the way I think it does.
My 5K iMac running Monterey — I’ve just shut down the M1 MBA so I can’t check it at this moment — doesn’t have Courier Prime, it has Courier New; I’ve never even heard of Cardiff or CrimsonPro; it doesn’t have OTF but it does have the full EB-Garamond set because I’ve installed that myself along with the complete Libertinus set.
Note, I don’t have Microsoft Office on any of my machines so I don’t have any of the fonts which that installs automatically.
Clearly I was wrong about Scrivener installing “Courier Prime” (I did it, obviously), but I’m still wondering: what does Scrivener do with those fonts in the program package?
I didn’t assume anything. Did you?
Thanks, but not really. Technically Scrivener “installs” those fonts, like in putting them on our disk. And maybe even using them somehow (?). But not in the Font Book (System Preferences).
How do you know? How can we tell the origin of an installed font?
This statement:
Assumes that (a) Scrivener is working properly, and (b) that you have a flawless understanding of Scrivener’s font handling under all circumstances.
Better to verify to make sure everything works the way one expects.
How would you do that?
Well, e.g. Cardiff, Crimson Pro and EB Garamond don’t show up there. At all. But Courier Prime does. But… now that you ask, those three fonts are available in Scrivener’s font menu! 
I’m lost.
No, they’re available in the system, and Scrivener uses a system dialog to reach them.
Can you please elaborate on this? What do you mean by “in the system”? I’m feeling a bit confused right now. Fonts installed there wouldn’t show up in the font book? 
Answered in the post to which you originally replied.