As a non professional it is always astounding how different the designs for injection molding are from hobbyist 3d printer parts. For 3d printing there so many

As a non professional it is always astounding how different the designs for injection molding are from hobbyist 3d printer parts. For 3d printing there so many caveats to make something easy to print. But for molds that seems to be on a different level. How you they now the plastic goes everywhere? Do they use flow simulations or is it experience?