I found that they are synchronized, if I delete a photo on iPhone, the copy on iCloud is deleted automatically. is it possible to only delete from the local device while iCloud never deletes something?
You can create a shared album. Add nobody to share with. So only you can see it. After uploading it to the shared album, you can delete it without deleting it from the shared album
I found that they are synchronized, if I delete a photo on iPhone, the copy on iCloud is deleted automatically. is it possible to only delete from the local device while iCloud never deletes something?
Generally speaking, you can not. The database for photos and the photos is sync'd across all your devices, they will all be the same.
iCloud Photos is a lossless copy of your photos and videos that are sync'd from iCloud to all your devices. You can optimize storage on your iOS device to keep just a very small version of your photos on your iPhone to mitigate storage concerns but anything you do to your photos on any of your devices will be sync'd to iCloud and to all your other devices. This includes deleting it.
Copying the photo to another location and deleting it from iCloud is an option if you don't want to loss the image but don't want to look at it either.
I believe there is a one to one relationship but if you are concerned about storage, you can use the "Optimized Photo" option. This will keep the full-res photos on icloud, while keeping a smaller optimized version on your phone.
I believe there is a one to one relationship but if you are concerned about storage, you can use the "Optimized Photo" option. This will keep the full-res photos on icloud, while keeping a smaller optimized version on your phone.