The census is mandatory, as others have noted. It is used for determining how many House seats each state gets, so it is important that it is accurate. And the Supreme Court has interpreted it to mean that an actual count has to be gathered everywhere, and that you can’t use statistical methods to derive the actual count.

So, even if they already have the information by some other means, they still need every household to give an affirmative response to yhe form. And if you ignore the form, they knock on your door to ask. While they may have other questions they want you to answer, you are obligated to tell them how many people live there, and their ages.

Visitors don’t need to be counted, but anyone with legal permission to live here does, regardless of citizenship.