I think the distinction between "what" and "how" is a matter of perspective. The "what" is definitely the domain of management (even if decisions are made with engineering input), but where the "what" ends and becomes "how" is fluid.
To use your bridge example, the "what" is "build a bridge" and the "how" is type of bridge, where beams go, etc.
That could just as easily be reframed as a more general "what" of "increase traffic capacity from A to B". The "how" might be a bridge, or it might be a new road that routes around the lake, a tunnel, or some other solution.
You could also frame it as more specific "what" of "build a suspension bridge" where the "how" no longer includes as many decisions.
You'd see the same distinction form between a micromanager and a hands-off manager. The former leaving almost all of the decision making in the "what" and the latter leaving almost all of it in the "how".