DMB, he didn’t start with “one/few million dollars”. That is merely the amount his father deliberately “lost” at his casino on one occasion, as a means of bailing him out from a jam he’d got himself into. The inheritance he left him was far larger that that.
It also helps if you develop a habit of stiffing suppliers and contractors.
And according to the ghost writer, Trump did not write a word of The Art of the Deal.