Only those individuals in a family who are applying for health insurance are required to provide citizenship and immigration status. Applicants also must provide a Social Security Number if they have one.
Citizenship and immigration status for those applying for health insurance will be checked electronically with several systems, including the Social Security Administration, the Department of Homeland Security, and SAVE (Systematic Alien Verification for Entitlements).
If an individual’s status cannot be checked through an electronic match, the individual can give other documentation of his or her status.
Federal and state laws protect the privacy of people who apply for or receive health care coverage or other public benefits. Under these rules, federal and state Medicaid and CHIP and Marketplace agencies may share information with other government agencies only for purposes of administering their programs, with limited exceptions. Healthcare.gov says on its website that information provided by applicants will not be used for immigration enforcement purposes.
You can browse related questions in the Marketplace Verification and Appeals section.