A highway can technically still be used for a freeway or interstate, because it essentially is a roadway that takes you from point A to point B. By the same token, a gravel road can be a highway because that’s what we used to have, or in some cases, still have as your main way of getting from place to place.
Meanwhile, the freeway is exclusively for cars and trucks and maybe a passenger bus or two because of the way it’s designed. One, for high speed; two, limited access. It’s not meant for you to take your bicycle on it or skate or walk, or go on a horse drawn carriage.