The game does not have an open world, but uses zones that allow you to move from one area to the next, which takes only a short time. On the world map all these areas that can be visited will be visible. The amount of visited and to visit areas will grow as you progress in the games. You can always go back to any area you visited before, so none of them will be locked. In some cases it is even a requirement to visit areas again to solve quests.
Nuclear Union allows you to have companions that follow and assist you. You cannot control these companions and they will use their own AI to assist you in combat. In the demo there was only one companion, but they did mention that the maximum amount of companions will probably be something like two, depending on further balancing. The companions can be selected from a larger group of potential companions you will encounter during your travels.