You stated you have nothing on your phone. Anyone here would take that to mean nothing is installed but iOS, and that it is somehow taking up all available space.
Your screen shot shows this isn't true. You have PUBG MOBILE installed, which alone is eating up 2.7 GB of the limited space on your phone. That and the 350 MB in stored photos. Despite that, your phone is NOT full. There's 1.6 GB of free space.
But the issue will always be the same. You only have 16 GB total to work with, of which iOS is taking up about a 1/3 of on its own. You just don't have much storage to work with.