How do I delete photos from my device and not from the iCloud?

This is not possible.

iCloud Photos is a sync tool not a storage option.

You have to turn off iCloud Photos on the iPhone, before you delete the photos from your iPhone. Then the photos will remain in iCloud and you can see them on a computer using a web browser at www.icloud.com

Or on a Mac or other devices using iCloud Photos.

If you want to save storage on your iPhone, but still use iCloud Photos on the iPhone, do not delete photos in Photos, but enable "Optimize iPhone Storage". Photos will remove original image files and replace them by smaller , optimized versions on the iPhone. It is fully automatic. You cannot control, which photos will be optimized to save local storage on the iPhone.