The portion is likely the operating system, as it should be, where it should be.
Just like a computer, the iPhone uses its internal storage/hard drive the same.
The apps & your saved content may (or not) also be included in total shown.
To delete or reset the iPhone, depending on how or what is done, may be
problematic. You could go through and remove apps; if they're visible.
How to Reset your iPhone (any generation) should not erase
..What model iPhone is yours..?
.My antique is iPhone 5C 16GB.
Good luck & happy trails! 🙂