In a total loss, irrespective of lease or buy, gap pays off any shortfall, I’ve never seen one that pay overage (ACV - settlement). So if the loan/lease still has a balance after insurance pays-off, it covers the difference.
It depends in the bank. For some, a new car loan has to originate at the dealer, some offer a loan with new car rates based on MY and mileage — e.g. Navy Federal has a “New Car Refinance” loan at the same rates as “New-new” if car is a 2023 or newer with up-to 30k miles