Refer to this:
Delete photos on your iPhone, iPad, and iPod touch - https://support.apple.com/HT205856 -
"Before you delete anything, make sure that you back up the photos and videos that you still want. When you use iCloud Photos and delete a photo or video on one device, it's also deleted on all other devices where you're signed in with the same Apple ID." That means iCloud too.
If you're short on space on your iPhone, read this -->
Note there's different kinds of "storage". Click on this link to "Learn the difference between your device and iCloud storage, ways to manage your space, and where to buy more if you run out." --> https://support.apple.com/HT206504
Take-away:
1. Storage on a device itself is fixed and cannot be upgraded. You have to change your present device for one with more capacity.
2. iCloud storage is on Apple's main computers. It is not a like a normal storage hard drive but by using optimization settings on your device it can help free up device storage. Lesser used items and larger file versions can be stored in ICloud and you can get access while connected to the Internet. It does not turn a 64 GB storage device into a 64 (or whatever) + 50GB/200GB/2TB device but if you have a lot of things that aren't being used much, or you don't mind only having lesser quality photos on your device it can free up extra space so you can make the best of the GB you do have on the device itself.