If you borrow against it you still have to pay interest. If he somehow found someone to loan him $1T, which would probably be practically impossible, the interest would make the total amount he got less than $1T.
if you borrow against it and buy shares of stock, the stockmarket generally can be counted on for at least 7% returns, and the borrowing cost would be 5%, so no, chances are the interest would not eat up the borrowing.
Buy-Borrow-Die resets the basis for capital gains tax, but then there's estate tax when the money gets passed on (exemption is only $15 million, trivial to billionaires).
No, taxed when you earn the money that repays the loan. Income tax, capital gains taxes, dividend taxes, estate taxes, etc... However the money was acquired to repay the loan, a tax was applied.
Can I e.g. borrow against my assets and not pay any taxes?
If you have a 401(k), yes. It's a way to turn a 25% credit card debt into a 5% loan.
The hitch is that while you can pay the credit card company over 30 years, the 401(k) loan is less than a decade, resulting in higher payments short-term, but money saved in the long term.
This strategy is not a traditional loan with interest and regular payments. If you try to live on regular loans it doesn't make any sense. It's a scheme mostly only available to HNW people where they repay upon death in certain tax loophole ways.
You won't pay taxes but you will pay interest. Most forms of value (real estate, gold, stocks, your car) you can borrow against in the USA, but the interest you pay almost always makes it a dumb idea.