Depends on the system. The older ones that are still most common are just screens facing up in the dash that reflect off the windshield. They are also nearly

Depends on the system. The older ones that are still most common are just screens facing up in the dash that reflect off the windshield. They are also nearly 100% blocked during the day for anyone wearing polarized sunglasses, which limits their usefulness. Newer HUDs use lasers and work with polarized glasses, but of course they are a more complicated, expensive technology.