How do I retrieve my iCloud photos from the cloud?

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 or Mac. 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

If you have iCloud Photos turned on then they should be automatically syncing with iCloud. You can verify by using an internet browser on any device and going to www.icloud.com and check in the Photos app there.