They're loans so where do they get the income to pay off the interest? If I attempt something like this via Interactive Brokers (which generally has the best rates), it would cost me 5-6%: https://www.interactivebrokers.com/en/trading/margin-rates.p... I'd figure if you're a billionaire, with multi-millions in collateral, the rate is probably significantly lower, but they still need to pay down the interest.
They borrow more, against other stock. Works as long as you have enough stock.