Yet another victory over MacOS!
To stop MacOS from rearranging your virtual desktops (after you have them just the way you want them):
Go to System Preferences > Mission Control
Uncheck “Automatically rearrange Spaces based on most recent use”
P.S. If virtual desktops (or “Spaces” as Apple calls them) are new to you:
Press “F3” to view your virtual desktops.
You can have multiple Spaces to group related windows and then swipe left & right between them using 3 fingers on your trackpad or magic mouse.