Having listened to 'good bad billionaire' for several episodes the formula is

1. Create or inherit a company
2. Do good at it
3. Float it on the stock exchange
4. Now own the bulk of a company whose stock values it at enough that your share is worth over a billion

Or

1. Make an insanely popular game
2. Sell it to Microsoft

Or

1. Be the child of someone who did one of the above