How do American billionaires avoid paying taxes?

Warren Buffet paid an 18% tax rate in 2007, while his secretary paid a 33% rate. In 2007, 2011, and 2018, Jeff Bezos and Elon Musk paid a total of $0 in income taxes. Every year, billionaires in the United States save $160 billion in taxes. How is it possible to be a billionaire while paying so little in taxes?