Uploading Albums from Photos to iCloud

I think we are confused by your saying " upload the Albums (directly as files) on iCloud.com." This conflates a couple of different ideas. What's confusing is that iCloud has lots of parts. There is a Drive that acts like a internet external drive with folders and files like are stored on our Macs with the Finder. If that is what you mean, then you would need to File>Export the pictures to new folders you create in the iCloud Drive folder listed in the Finder sidebar. It would be easier to use a program like PowerPhotos to export albums for you-- it can export entires folders and keep the album structure. (This will take time, maybe days, to finish.)

The app Photos doesn't usually work with "files" in the Finder sense-- It stores the pictures with crazy seemingly meaningless names in a user not-safely-accessible "package," and uses a clever database to keep track of it all. That's why when you use Photos, you use the Export dialog rather than finder-like dragging or copying.

Another option is to use Preferences>iCloud to enable iCloud in Photos (with only one Mac Library, which you choose by identifying it in Preferences as your System Library, as léonie has said.) Then that entire library is copied to the Photos app in iCloud.com with exactly the same structure. This isn't a backup, it's a synchronization. If you delete a picture on you Mac, it will be deleted in iCloud. iCloud Photos will also automatically copy pictures from your iPhone and iPad if you turn on iCloud for Photos on those devices. Backups are kept on external drives (not iCloud) using Time Machine or Carbon Copy Cloner or some other reliable backup system.

Another option, which is very rare, is to have Photos refer to picture Files in Finder folders rather than keeping them in its own package. It's very easy for Photos to lose track of the pictures this way.

So you have to be sure which of these things you want. We can help if you need it.

All you need to do is enable iCloud Photos. iCloud Photos works with the Photos app to keep your photos and videos securely stored in iCloud, and up to date on your iPhone, iPad, iPod touch, Mac, Apple TV, and iCloud.com: Set up and use iCloud Photos - Apple Support.

Jack

If you have several Photos Libraries, only the photos and albums from one of your libraries can be syncing with iCloud, the Photos Library that you make your system Photos Library (System Photo Library overview in Photos on Mac – Apple Support (UK)).

Keep the photos and albums you want in iCloud in your System Phots Library, all other photos and videos in a different Photos Library. (https://support.apple.com/en-gb/guide/photos/pht6d60b524/mac)

Enora,

Some things about iCloud Drive:

You can't fully access a Photos Library on iCloud Drive. Photos needs the Library to be connected by a wire, not a network. So Photos Libraries on iCloud Drive act only as a backup.

iCloud Drive behaves like a folder on your Mac, not like an external drive. So, like in Finder, if you drag a file from one folder to another, the file disappears from the original folder. It's moved, not copied. So if you drag a Photos Library to iCloud Drive as a backup, it will be removed from your local drive. I have been confused by this more than once! So, like in Finder, if you want to copy your Photos Library to iCloud Drive, you need to Option-Drag the Library to iCloud Drive.

Because Photos Libraries are huge, using iCloud Drive can be surprisingly slow. It may take days to copy a Photos Library to iCloud Drive. A program like Carbon Copy Cloner, after the first long transfer, updates only with changes that have been made since the last update, and so is much faster. (I haven't used CCC with iCloud, only attached external drives.)