When you are good at the fundamentals and then put in a position to succeed using them, you have confidence. But it helps to have superior innate abilities, and is also partially just how you were born.
As for having the ball in his hands, he'll get plenty of opportunities. Even GRIII, who had a lot of off-ball/residual action offense, had the ball a lot. He basically had as many shots as Nik and Caris in 13-14, with lots of opportunities to rebound and go, get the ball at the elbow off curls, etc. Matthews looks to have a better handle, but we'll see. He won't be the guy with the ball like Trey often was in 12-13, but he'll have the ball plenty.