I would back up the phone and then erase all content and settings and re-measure it with no user apps or data or things installed - just the blank OS.
If you just want to poke at things before erasing, I would put the device in airplane mode then do the power off ( slide to power off in screen - controller shut down of iOS). Then start the device and open the storage viewing app. Ideally you connected to power and let the spotlight index rebuild for 15 minutes on power and in airplane mode (so no new data arrives on device)
Then if it's still too high (or you're just curious how much thinner you can make it), restore the device from iTunes (perhaps in restore mode).
Something is really off with that large an amount. I expect 3 GB of system space on older iOS versions and older devices:
Not the whopping 50 GB you post in your image. Either there are backup snapshots or something else really taking up space. If you only have 64 GB of space, I could see the device barely operating if it's reserved that much space. Even a larger storage device could be in trouble, so back up and then clean all your apps / settings and consider a restore for good measure.