First, nobody really needs a billion dollars in "cash" it just doesn't make sense as "cash" deposits pay a below inflation rate return in most places (meaning it loses its "value" over time) and even a negative interest rate in some places.
Second, it isn't like there is a "fixed amount of money" in the world and if someone has all of it, everyone else has none.
So if you are a billionaire, and most of your billions are in stock of a particular company, and you are a US citizen. You file a form with the Securities and Exchange commission (SEC) to sell some shares every month or quarter or year. Or you file a form that says you will sell X shares 3+ months from now. The SEC posts this information publicly so that other investors will know about it ahead of time and decide what to do. The point being that you are a "big investor" or an "insider" and selling might be seen as a signal, and this makes the signal loud and with a time delay so that the other investors can respond. Then the appointed time comes and your shares get dumped on the market and you collect however much money they happen to bring in (less taxes).
You will see lists like this one (http://www.nasdaq.com/symbol/goog/insider-trades) for Google. Which itemize those sales. So lets say you need 15 million a month to operate your private jet, you set up a system that will sell enough stock to net at least 15 million a month to cover that. You don't want to sell it all since you might want to vote your shares but you have to get around so you let some go.
A lot of tech billionaires set up trusts for their children which they fund such that their children will never have to work and can live a comfortable lifestyle. A number of tech billionaires have been setting up foundations (like the Bill and Melinda Gates foundation) which can convert stock to cash in a tax advantaged way and do good things with it. Sometimes the foundations are created to give the billionaire a vehicle for influencing others while nominally keeping their hands clean. (not working so well for Trump and Clinton but I read about a number of foundations being used in this way.)
So generally if you look at the insider trading you will see billionaires selling stock in 10 to 100 million dollar chunks and using it for different things.