christopherfromemsworth wrote: … Photos from elsewhere in iCloud seem to be stored in a 'recently saved' folder.
Pictures are not stored in the "'recently saved' folder." In fact, pictures are not stored in any folder or album. The Library view shows all your pictures. And other views, as markwmsn said, offer ways to see selections of pictures from the Library. A picture you see in an album, in Recently Saved, in Favorites, or wherever, can also be found in the Library view. In the most recent iOS, you can filter the Library view to select certain classes of pictures, and you can sort the view by capture date or by saved date. (So, if you can't find a picture in Library, you should check the filters and sorting.)
About synchronization: all the pictures with all their information is stored at iCloud.com, and each connected device has access to all that information. How pictures and info are presented is constrained by the properties of each device. Macs work with the pictures with the advantage of a keyboard and mouse, while iPads and iPhones use touch screens and typically have less computing power, battery life, speed, and consistent internet access, so it's not at all surprising that they deal with pictures differently.
If you're having problems with WhatsApp, then you need to talk to those people. WhatsApp isn't an Apple product.