That's not him saying how he'd implement it, anyway. He has no stated plan for how he'd make Mexico pay for the way, no timeline/plan for how long it will take to build, how much it will cost, the specs of the wall, or really anything at all.
You forgot that he actually knows all of this and has detailed plans himself, you know, because of his very good brain and the many things he's said. But he will call in "the best" people and have them submit their plans too...I'm not sure why though, because he's already said that all his plans are the best and he knows more than literally everyone else, anyways.
Or off the spot, for that matter. Nor has his campaign.
Well, he sometimes will get into vague details like he did with his tax plan...which he changed within a matter of hours to suit whomever he was speaking to.