The census is described as the largest scientific undertaking by the federal government, outside of a military operation.
So how does the U.S.Census Bureau fulfill its constitutional mandate, to count each person in the country and associate them to an address? Very carefully one could say, using multiple strategies to identify individuals who might be between homes, persons not at their primary home (college students, or those incarcerated, in the military etc. but the key operation is the questionnaire mailout whereby questionnaires are mailed to homes and mailed back in.
2010 Census Website
NYTimes Article: The Importance of being Counted
(See the process explain in the video below)