how do I restore my System Photo Library from the photos stored in iCloud on a Mac desktop?

If your Mac is set up to use iCloud Photos, it will attempt to synchronize its System Photos Library with the photos stored in iCloud Photos on Apple's servers. If your Mac has enough free storage space, you may want to set preferences so the Mac will try to keep full-resolution copies of all of the photos in iCloud Photos.

Set up and use iCloud Photos - Apple Support

I see there is a Related Article link to "If My Photo Stream isn't updating."

My Photo Stream is going away. All uploads to My Photo Stream stopped late last month, and the My Photo Stream service will disappear forever in a few days (when the last of those uploads would have been deleted from the photo stream anyway). If you have been using My Photo Stream to get (reduced-resolution copies of) photos from one of your devices to another, your window to copy what few photos are left in the stream is closing rapidly.

Information about the My Photo Stream shutdown - Apple Support

iCloud Photos – which doesn't automatically delete photos after 30 days – is a different service, and it is not going away. Both were Apple services that worked with Apple devices and the Apple cloud, but that's about all that they had in common.