Mirror some displays but not others
The Alt key is a Windows term for the alternate function key, in this case macOS calls this the option key, with the symbol ⌥.

It sits on most keyboard layouts between the control and command keys to the left of the space bar, and between the arrow keys and the command key on the right of the space bar.




  Note: On a Windows keyboard connected to a Mac it is likely the Windows key will act as option as these two keys are swapped in most layouts.


I can confirm the steps mentioned do work for mixed mirroring and extending of displays, although I am unsure if this works with Sidecar.