The new King wants a large, technically advanced, solar-powered, very wired, secular, self-governed, modern city, at the confluence of four regional states -- the KSA, Jordan, Egypt, and (though not mentioned) Israel. A lot of the land area will be covered with single- or 2-storey residential and other structures, hence the sprawl (33 times the area of NYC).
It would be nice if he could talk the Egyptians into it instead, building it in the Sinai peninsula. Ah well.
And just WHO is going to live there? The population of Saudi Arabia isn’t enough to fill a city that gigantic.