why there is a diagramm on how to navigate it

You don’t need a diagram after undergoing years worth of training at the Starfleet academy

But the gap between what Okuda did and what we see on phones, tablets and the general web today are worlds apart.

A phone is a general purpose device. A Star Trek terminal, or any purpose-built interface will have dedicated buttons for the things it needs to do, so that those things can be done quickly.