iCloud back up
How do you erase photos on your phone but keep them in iCloud if every time you erase something from your iPhone, it erases from your cloud?
iPhone 8, iOS 13
You cannot because iCloud doesn't work like that or designed as such. If/when you delete photos off iCloud or on your device, they are deleted everywhere -- all devices and in iCloud too. iCloud is a syncing service that basically syncs all of your data across devices.