deleting photos from an iPhone

Delete photos on your iPhone or iPad - Apple Support - "Before you delete anything, make sure that you back up the photos and videos that you still want. When you use iCloud Photos and delete a photo or video on one device, it's also deleted on all other devices where you're signed in with the same Apple ID."

Get help with iCloud Photos - Get help with iCloud Photos - Apple Support

Go to the section "How do I turn off iCloud Photos on only one device?

You would have to disable photo syncing on the device first and keep it off, otherwise they would be deleted in iCloud too.