What are the rules around medical care costs for visitors to the USA who contract Coronavirus?

What would be the situation for a visitor should they contract Covid-19 and require hospitalisation whilst in the US?
Would it be classed as an emergency or secondary care? Does the US provide emergency care free of charge?
Do charges vary e.g. by state or hospital?
My question was prompted by this one Is it safe for an ESTA waiver visitor to return to Japan until the corona virus has been contained?, which made me wonder about the financial risks of extending a visit, given that many travel insurance policies limit the length of each trip. For example, I have an annual travel policy but cover is limited to 45 days. It also excludes any epidemic or pandemic.
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