Incorrect. It's at-will employment.

"At-will employment" is a meme here. In reality, the company gives you a few months of pay and you sign a document saying you won't sue them for firing you during a layoff.

If you get fired for fraud or for being incompetent, for example, it's often different.

That's the convention, but AFAIK it's not enshrined on law.

Some states require payout for unused, earned vacation time.

State-managed unemployment pay is also a thing, assuming the employee wasn't fired for cause. I think some states require employers to pay into this via a payroll tax.