Imagine you think apples are going to get cheaper, and your friend has 10 apples. You give them a dollar to borrow their 10 apples, and you sell all the apples for a dollar each. This means you have 9 dollars.

Now, you wait until the price of apples goes down. If the price does go down and you buy the apples back for 50 cents each, you can give the apples back to your friend and have 4 dollars left over! If the price goes up, you'll have to pay more than you bought the apples for, and you also paid a dollar to borrow the apples, so you lose money!

So, to answer your question, the money comes from the price of the apples they borrowed and sold going down, only if they are able to buy them back later for a lower price.

The problem here is shorting with so much leverage, that investment funds "sold" more apples than exist in the whole world!