Everyone over 65 is eligible for Medicare, regardless of income, but others can qualify under some circumstances.

In 2022, according to the 2023 Medicare Trustees Report, Medicare provided health insurance for 65.0 million individuals—more than 57 million people aged 65 and older and about 8 million younger people.

You have to qualify for Medicaid, and I think different states have different requirments.

Medicaid is the largest source of funding for medical and health-related services for people with low income in the United States, providing free health insurance to 85 million low-income and disabled people as of 2022; in 2019, the program paid for half of all U.S. births.

There is also CHIP, and I think that is part of Medicaid.