How to Stop getting photos synced
How do I keep my iPhone photos from showing up on my iPad?
iPhone 6 Plus, iOS 12
I'm guessing that you are using iCloud Photos, which would sync your photos to all of your devices. To check, go to Settings><yourname>iCloud, then Photos, and see if iCloud photos is turned on for either or both devices. You would want to turn that off. If you are using iCloud photos, remember that deleting a photo on one device, deletes it from all devices. See this support document, Set up and use iCloud Photos - Apple Support