The WARN act dictates that you need to provide 60 days notice for certain mass layoffs. Typically this means you are laid off, but remain on payroll for 60 days.
No severance is not apart of the law. You can be laid off and fired at anytime for any reason without severance. My manager laughed in my face when I asked if there was severance when being laid off in GFC.
Some states even have laws that employers don't have to pay accrued vacation time. For example Nevada says employers with under 50 employees don't have to pay accrued vacation.
As mentioned in another comment there is the WARN act[1] at the federal level. And many states have additional regulations. In practice, employers would often prefer the people laid off just stop working immediately than continuing coming to work knowing they are losing their jobs soon. And I think employers can offer employees a severance agreement where the employee waives their right to the 90 day notice in exchange for some other compensation, such as a lump sum payment.
However, there are exceptions. In particular if the company is small enough, or the layoff is below a certain percentage of employees, it doesn't take effect.
