I'm not familiar with the US laws. But I have seen a few cases where the terminated employee sued their former employer for not giving them the severance pay they were owed. That's why I made that (possibly wrong) assumption. So, how does that work?
If you mean how does that lawsuit work, the only way the employee wins is if there's some type of agreement that says they should get paid. That agreement could be employer's written policy, an individual or union contract, an email from someone with authority, or so on.
(Or the lawsuit could be a hail-mary on its merits, hoping the employer will settle rather than air dirty laundry in court.)
There may be state-specific laws (in a small number of states for limited circumstances) and there is the WARN Act (but any court payout there would be a penalty on the employer and much delayed for the workers, while it only requires advance notice rather than being like severance -- and also limited by more conditions), but still "no general entitlement to severance" for the vast majority of workers no matter the reason for their separation.
