Where do millionaires keep their money if banks only insure $250k?

I'm curious, for those with a significant amount of wealth exceeding the traditional $250,000 bank insurance limit, where exactly do they keep their money to ensure it's both secure and accessible? Is there a specific type of financial institution or investment vehicle that these millionaires and billionaires favor for safeguarding their vast fortunes? And what measures do they take to protect their assets from market volatility, inflation, or other potential risks?