That is how you short a publicly traded stock. The mechanics of the shorting in the Big Short is actually a lot different. It is done with credit default swap

That is how you short a publicly traded stock. The mechanics of the shorting in the Big Short is actually a lot different. It is done with credit default swaps which you can basically think of as an insurance policy. The difference is that it is on a financial asset instead of on a physical asset and you also don't need to actually own the asset you are insuring. Like buying insurance on a car you don't own (you can't actually do this). Pay the premiums and if the car crashes you get the value of the car in cash.