How do I keep photos on only 1 iPhone? I have a old iPhone 10 and want my pictures only on the 10,?

My current iPhone 11,shows my memory full. I’ve had this phone since June of this year. Is it possible I have a defective phone already? My old iPhone X ,has the same amount of pictures as my current phone. How can I get the proper use out of this iPhone 11? I was told by the young man at T-Mobile, I need to take my pictures off my phone because that’s the issue.