It does need more elaboration, but using loans with stock as collateral is a way to avoid, or at least minimize, taxes very long term. If the collateral raises in value at least as fast as the interest rate, you can take out more loans to service the initial debt. If the collateral raises faster than the interest rate, this can be extended to add on more debt load in total. Using this is risky, in the same way that margin stock is dangerous: losses are magnified more than gains are, and a stock drop can wipe out many times the amount of wealth from the owner.

The loans will also let them do better timing with sells, delaying payments such that they can liquidate a stock that lost value at the same time as stock that gained value, reducing the taxable liability when they do actually sell. This is presuming they hold any stock that actually lost value.

Most of the rich will have outside sources of income that they can use to service any debts they have. This will include items such as base pay and stock grants from the companies they own.

If they manage to delay the repayment until death, proper estate planning allows other tricks with minimizing tax liability.