How do I see photos in my iCloud on my phone
How do I see photos that are stored in my iCloud through my iPhone
If you are using iCloud Photo Library, you view the photos via the Photos app on the phone. They are the same as in iCloud. More information: iCloud Photo Library - Apple Support
You can also use a computer and log into www.icloud.com and view them there. The iCloud website is not designed for mobile browsers but you can force Safari on your phone to access the site. First attempt to go to www.icloud.com in Safari and then press the sim-circular refresh button in the Safari address bar until "Request Desktop Site" appears and then tap that. The results will not be great.
If you don't have iCloud Photo Library turned on, photos would be included in an iCloud Backup but you cannot view the contents of the backup. It can only be used to do a full restore of a device or to set up a new device.