You still need to pay loan up with real money on which you pay income tax.Whatever they spend they earn as taxable income.
No, they don't.
They spend the proceeds of loans against their portfolios.
And I seriously doubt Bezos pays his bank $500/month for his boats.
Bezos has to have at least $30mil/year income to support his lifestyle. This moves him well into 37% bracket.
You probably don't need to pay much if anything in the way of principal on a margin loan, at least if the underlying stock is going up.
And Bezos could fund his $30mil/year income for 6300 years on sales of his stock, on which the top marginal rate is 20%...