Your photos are always available on both iPhone and iCloud. You delete it on iPhone it is gone from iCloud and vice-versa. To save space on your iPhone use Optimised iPhone storage. Remember, if you delete in one it is deleted everywhere.
Get started with iCloud Photos:
- Tap Settings > [your name] > iCloud > Photos.
- Turn on iCloud Photos.
- Select Optimize iPhone Storage to save space on your device.
With Optimize Storage, smaller, space-saving photos and videos are kept on your device while all of your original, full-resolution versions are stored in iCloud. And as long as you have enough space in iCloud, you can store as many photos and videos as you want.