Photos from Mac to iCloud

When photo library and its preferences is set up Set up and use iCloud Photos - Apple Support

Change preferences in Photos on Mac - Apple Support

System Photo Library overview for Photos on Mac - Apple Support

If the photos that are kept on desktop screen , drag and drop pictures in the photo library folder they uploads to iCloud.com as well in photo library .

You don't need to download them from iCloud.com , they are in server .

The purpose of iCloud Drive is to save personal files / personal documents Use iCloud Drive on iCloud.com - Apple Support not to create a folder in finder > iCloud Drive and save photos in it , definitely photos will be saved in that folder , the purpose of iCloud storage is explained What's in Others in your iCloud storage - Apple Support

What you can do with iCloud storage - Apple Support

The storage didn't changed as these photos were in system photo library folder of photo application of Mac and in iCloud.com

System Photo Library overview for Photos on Mac - Apple Support

Even your Mac is corrupted and you erase the hard drive and reinstall Mac OS photo that are saved on iCloud server will never be lost , also if photo application is corrupted see some articles Back up the library in Photos on Mac - Apple Support

Repair your library in Photos on Mac - Apple Support

But don't save photos in iCloud Drive .

Do I understand you correctly?

You are keeping your photos and videos in files and folders on iCloud Drive and are not importing them with Photos into a Photos Library that is syncing with iCloud Photos?

If your Mac is lost or stolen or your house should burn down, you can get all photos back from iCloud Drive that you stored on iCloud Drive. and you can get all photos back, that you stored with Photos in iCloud Photos Library. iCloud is great as an off-site storage and also for syncing across devices.

But you should also make a local backup copy, so you can access your photos, if the network should not work for a longer time, or to recover accidentally deleted files. iCloud does not offer an archival backup for users. We can only access the current state. If you accidentally delete photo and only notice it later, when the photo is no longer in the recently deleted items, you cannot recover it from iCloud. You need a backup to protect yourself against user errors. See: Archive or make copies of the information you store in iCloud - Apple Support

Ou can check what has been uploaded to iCloud Drive on your iCloud web page at www.icloud.com .