Only to survive in today's economy, having a side hustle is advice, not to talk more about becoming a millionaire, you have to have a diverse income(businesses) that will bring you huge returns.
No millionaire does have a single source of income. They have many sources of income, and when they see any financial opportunities that will pay, they don't miss it. They are all ready to have a risk of investment in it with huge money. If they fail, fine but if they don't they will huge amount of money