how to delete photos on iPhone, but keep on iCloud?

iCloud Photo Library is a syncing service. Any photo you delete from a device where iCloud Photo Library is turned on will also be deleted from iCloud and from any other device connected to that iCloud Photo Library. You can potentially reduce the storage space required on a device by turning on Optimize Storage. See: iCloud Photo Library - Apple Support

Apple does not currently offer cloud-only storage for photos. If that is what you want or need, you may want to explore 3rd party options such as Google Photos or Dropbox.

Another way that photos may be on iCloud (if you don’t have iCloud Photo Library turned on) is via a backup of an iOS device. The only thing you can do with a backup is use it to restore a device or set up a new device. The contents of backups are not viewable at iCloud.com and if you delete a photo from a device it will not be included in future backups and will be lost.

If you have the means to do so, I recommend importing the photos to your computer and then back them up to an external drive as well. Redundant backups are a good thing.

Ignore the iCloud information. Using iCloud Photo Library is what you want to avoid if you want to safely backup your photos.

Transfer photos and videos from your iPhone, iPad, or iPod touch - Apple Support