they don't count tourists. Or at least, not foreign tourists. if you live in the US, you're obligated to be represented on the census in some way- every household is expected to fill it out once, so if you live with your parents, Also you can chose to not answer questions, but they'll do what they can to fill them in. (including potentially making shit up.)

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 the 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 only obligated to tell them how many people live there, and their ages.

(Edit: I looked it up, and I was wrong. They send a longer form to some households at random, and if you get that, you are technically obligated to answer all of those questions as well. )

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

If you ignore the thing in the mail, they send a person to your house to knock and ask the exact same questions.