What tool does the government use to determine ethnic breackdown of United states?

The United States Census Bureau uses the American Community Survey (ACS) to determine the ethnic breakdown of the United States. The ACS is an ongoing survey that collects data on demographic, social, economic, and housing characteristics of the U.S. population. The survey asks respondents about their race, ethnicity, place of birth, and other identity-related topics. The results of the ACS are used to produce population estimates and projections, as well as to inform public policy and decision-making.