Do you mean "iPhoto" which was discontinued 10 years ago? The extension implies that this is a Photos Library, the current photograph management system. So is this an updated version?
Each account can have only one Photos Library at iCloud.com, and a Mac can have only one Library connected to iCloud and thus to that iCloud Library. iPhones and iPads can connect to only one account, and they have only one Photos Library.
The usual idea is that iCloud Photos is a synchronization service. When you engage iCloud Photos on a device, then the Library on that device is kept exactly the same as the iCloud Photos Library, and vice versa. So, for instance, if you take a picture with your iPhone, it is added to the iPhone's Photos Library, copied to iCloud Photos Library, and then copied to the Photos Library on each of the other devices that you have connected. If you delete a picture on your Mac, then that picture is deleted at iCloud and on all the other devices. And, if you delete a picture on iCloud, it will be deleted everywhere else. That's what "synchronization" means.
If you want the pictures "saved in the iCloud only (i.e. not in any of my devices)" then none of your devices could be connected to that Apple account. You would see it only when you signed in to that account at iCloud.com. On the other hand, if you want to "see it directly in the Photos app," then that app would need to be connected to the account. And pictures added to that device would be added to the iCloud Library and copied everywhere else.
You could have a special second Apple account for just this Photos Library. But that would cost more, of course. On a Mac (but not iPhone or iPad) you can have two users, each connected to a different account.
It sounds like you're not really going for synchronization, which is what iCloud is about. iCloud is a dynamic system, designed for Photos Libraries are constantly changing. So it may be that iCloud won't work for you for this purpose.
