Using Referenced Photos To Exclude Photos From iCloud
If you wish to have some of the photos on your Mac available in iCloud, but not others, a typical solution is to use two Photos libraries. But a more useful solution for some is to link some photos as referenced photos, keeping them separate from your Photos library while including them as linked photos. These items will not be synced to iCloud and will not be stored in your Mac's Photos library either. You can bring these photos into the library later on using the Consolidate command.