iCloud Storage

Hi! I'm not so great with technology so this may be a really elementary question, however I'm confused about how iCloud works. Both my phone and my computer are completely backed up to my iCloud and I've paid to have extensive iCloud storage. On top of this, my phone and my computer are reaching their maximum capacity for storing my photos, documents, information, etc. I would like to delete these from my devices whilst having the security that the could be accessed in my iCloud. However, I'm having issues deleting the items from my devices and keeping them in the Cloud. For example, when I go to delete photos from my phone, it tells me that deleting the photos will "be deleted from iCloud photos on all devices". How do I change this? Any help would be appreciated, thanks!