How do I view my photos and iMessages which have been backed up on iCloud

If you have iCloud Photos turned on, the photos on the device are the same as on iCloud, with the possible exception that some of them may be space-saving optimized versions if you have that option turned on. You view them in the Photos app on an iOS device. You can also login to icloud.com and go to the Photos app there. See: Set up and use iCloud Photos - Apple Support

If you didn't have iCloud Photos turned on but do have an iCloud backup, you can restore the entire phone (not just the photos) from iCloud but you cannot otherwise view the contents of the backup. Restore your iPhone, iPad, or iPod touch from a backup - Apple Support

Also see the following from Use Messages in iCloud - Apple Support

You can use Messages in iCloud on your iPhone, iPad, iPod touch, Apple Watch, and Mac. For your privacy, Messages in iCloud is end-to-end encrypted,* which means you can't view or access Messages online via browser. 

You cannot view the contents of a backup. However, if you are using iCloud Photos, you will see them on your device, since using iCloud Photos is a sync service, where photos are synced to all devices logged in with that Apple ID and has iCloud Photos turned on. You can also log into www.icloud.com/ and view your photos on a browser. As far as Messages, if you are using iCloud Messages, those are synced across all devices like photos if you have Messages turned on in Settings. I'm not sure if those appear in the browser when you log into iCloud, as I do not using iCloud Messages.