How do I access the pictures I have stored in iCloud that are not on my phone anymore?
How do I upload old pictures from iCloud
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If you have iCloud Photos turned on, the photos on your phone 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 use a computer and login to icloud.com and go to the Photos app there. See: https://support.apple.com/en-us/HT204264
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. https://support.apple.com/kb/ph12521