How to use MacOS Cmd -F1 screen mirroring shortcut??

Where in the settings does it map cmd+F1 or cmd-fn-F1 to Mirror Screen or whatever it is called.

I found it, here: Mac keyboard shortcuts - Apple Support

I'm trying to figure out what the OS thinks that function should do.

It states:

Command–Brightness Down: Turn video mirroring on or off when your Mac is connected to more than one display.

To me, that means it does exactly that. It doesn't say it enables "screen mirroring" which has the capability to extend the displays.

In the macOS User Guide, it describes the two functions separately:

• • You can use video mirroring to show your entire desktop on each display.
  • You can extend your desktop across two or more displays, and choose which display shows the Finder and application menu.

So, it appears that "video mirroring" does indeed mean "video mirroring" and not "video extending."

Where is that shortcut defined. I could not find it.

When I use the Control Center to enable mirroring, it uses the same settings as I previously used for that display.

Your description sounds like it is turning on mirroring for the first time. Maybe that's what that shortcut does. It isn't enabled for me and I couldn't find any way to enable it.

Does yours remember the settings if you enable it from the Control Center?

But where is it defined? It appears to be a shortcut for "mirror exactly," not the screen mirroring you get from the Control Center and Displays System Settings.