how can I delete photos on my phone but not have them deleted in iCloud?

If you are using iCloud Photos, when you delete a photo on your phone, you delete it in iCloud as well. iCloud is merely a syncing service, not off device storage. You can download the photos to a computer and then delete them from the phone if you want to make room on the device.

You can read more about iCloud Photos here.

Set up and use iCloud Photos - Apple Support

You can read about archiving photos to your computer here.

Archive or make copies of the information you store in iCloud - Apple Support

While iCloud sync is enabled, you can’t delete photos on your device without also deleting them from iCloud. However, you can disable iCloud sync altogether and then delete the photos from your phone.

Also, consider having your device store low resolution local copies, while saving the full resolution images on iCloud. You can enable this by going to Settings/Photos and tapping “Optimize iPhone Storage.”