Suppose you win 100 million. What do you actually do with it? Banks only guarantee 250,000. Do you have to invest it? Is there anywhere you can just let it sit and draw interest?
Either you or someone with a fiduciary duty to you will open up several accounts to invest in your name. These accounts will vary but be geared to making you money, providing the liquidity you need, and hedging against market risk. You might also make an investment in illiquid assets like real estate or hedge funds.
It won't be guaranteed unless you pay insurance on it, but you should be rich enough to take the hit of one account going under.