If the desktop is a Windows PC and you are using iCloud for Windows, note that only photos taken after iCloud for Windows is set up will automatically appear. You can manually download older photos to the desktop (if they are in iCloud Photos). See the following from Set up and use iCloud Photos on your Windows PC - Apple Support
New photos and videos you add to iCloud Photos will automatically download to your PC. You can also download your collection by year when you follow these steps:
- Open a File Explorer window.
- Under Favorites, click iCloud Photos.
- Click Download photos and videos.
- Choose the photos and videos you want to download and click Download.