Most of these young “billionaires” are the result of successful money laundering
You ask "How did they start?".
You hear, "He started by owning one big hotel then spread tentacles"
Ehn we know... How did he start that big hotel....*Silence*
Once you’ve successfully made money through a repeatable, non-random process, making money again often becomes easier because now you have both capital and a mental model for how money is made. It’s actually a game