How do I connect iCloud to my phone?

I pay for 2 TB a month on iCloud yet my iPhone 7 plus keeps saying my storage Is full so I keep having to delete stuff to free up space. My phone has the capacity for 128 GB — I have nearly 50,000 photos on my phone - it will take days to transfer everything onto my laptop and will probably cause all kinds of tech issues that I can’t be dealing with right now. How do I link the iCloud I am paying for to my phone so that my stuff is stored in the cloud like everyone else??