Do you really believe that someone would loan $1B backed by $1B in stock gains that haven't been taxed? Yes, you can borrow against stock, but the lender won't

Do you really believe that someone would loan $1B backed by $1B in stock gains that haven't been taxed? Yes, you can borrow against stock, but the lender won't give you full value because the stock can go down and they will account for the taxes on unrealized gains. Not to mention that lenders do want their money back. At some point, stock will have to be sold to cover your expenses (including interest on your loans) and that's going to result in taxes paid.