For folks with Amazon Prime (or Amazon Drive), the Amazon Photos app can also automatically back up photos onto your cloud and keep thumbnails. Folks with Prime have no limits on storage for photos at full resolution. If you upload to Amazon Drive and then delete the photo from Photos, then the thumbnail will still be in the Amazon app. The thumbnail there and the photo on Amazon Drive are coupled together (so delete does both). I'm not sure if the app tries to keep thumbnails of everything. For many thousands of photos and small capacity on an iOS device, that would still be space.Since Google Photos is a straightforward setup on iOS, one doesn't have to use an Android to take advantage of their photo storage scheme, if it works better for you than iCloud Photos.
So, different apps have a different focus.