Multiple current, and more significantly former SpaceX engineers have confirmed Musk was the driving force behind the engineering decisions that lead to reusability for Falcon 9.

He’s also been very much in the driving seat on engineering for Starship, and we’ve yet to see how well that works out, but the success of F9 is there to see.

There are also stories from engineers at both SpaceX and Tesla that they do everything possible to keep him away from any engineering decisions because he doesn't know what he was talking about. His brainchild of the cybertruck was suppose to be a monolithic bent stainless steel shell that he spent years trying to accomplish and fired many engineers over who said it wouldn't work, and we see how that turned out with strips of low quality stainless glued onto aluminum parts that are suppose to be load bearing with a regular steel undercarriage that will still rust away.