When you organise your Photos Library with Photos for Mac, and create albums and folders, these albums and folders will sync with iCloud Photos to all your devices. I am seeing the same folders and albums on my Mac, my iPad, my iPhone, and also at www.icloud.com in Photos. We just cannot create albums directly on the web page, but we can see them, once they have been created in Photos on one of our devices. The faces, the titles, the captions, the keywords, the locations will also sync with iCloud, also the adjustments we apply. The only thing, that is not syncing with iCloud, are the projects. You will have to recreate the books or slideshows on other devices, as the projects will not be uploaded to iCloud.
The ability to sync the structure of our Photos Library across all our devices is the main reason, why I like to use iCloud Photos. I am organising my library on my Macs, and can enjoy the albums and folders on all devices. One caution - the titles and keywords are also saved in iCloud, if present, but the iPhone cannot show them, only your Mac. We can only see the captions on the iPhone, and only since iOS 14 or iPadOS 14. So, if you want to see some text with your photos on an iPhone or iPad, add the text not just to the titles, but also to the captions.
If you want to keep backing up the Photos Library on your Mac with Time Machine or cloning software, do not enable "Optimise Mac Storage". This will remove some of the high resolution originals from the Mac and keep them only in iCloud to save storage on your Mac. The optimised items will be missing from your backup, and you will no longer have a backup of all originals. If you use "Optimise Storage", you will have to backup the originals separately, by exporting them. This is a lot of work and error prone.
