You cannot access the iCloud.com website directly from an iOS device. You can tap on the Share button at the bottom of that screen and then navigate to the Request Desktop Site option to open a limited version of the site, but iCloud.com is designed to be accessed via a browser on a computer for full access.
Your photos and videos in the iCloud Library are found on your device in the Photos app. If you have Optimize Photos turned on, then you may not see all of your full-resolution photos on the device, but instead, you might see thumbnail versions instead. You can tap on any thumbnail which results in the full-res version for that photo to be downloaded. But you only have thumbnails if you are low on memory on your device. If you have enough memory available on your device, then what you see in the Photos app under All Photos is the same thing that you see in iCloud.com under Photos.
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