To understand the U.S. health insurance field, three concepts need to be clarified: Medicare, Medicaid, and ACA.

Medicare:
The U.S. federal government's "federal health insurance" for the elderly and some people with disabilities: a universal health insurance designed by age/disability eligibility, not by income eligibility (that's Medicaid).

Medicaid:
Chinese "Federal Medical Assistance," a "medical assistance" program for low-income and vulnerable populations in the U.S.—jointly funded and managed by the federal and state governments, providing health insurance and medical services to people with limited income and assets.

ACA:
The Affordable Care Act (commonly known as Obamacare). It provides a safety-net medical coverage mechanism for people who do not have commercial insurance, have low or unstable incomes.

The specific operation is "for people whose income falls within a certain range, if they are not covered by employer group insurance or the Medicare/Medicaid system, they can purchase insurance through the health insurance exchanges established by the ACA and receive federal premium subsidies, preventing them from being completely uninsured due to high premiums."