Can people really get billionaire just by investing money in stock market...

No you have to know when to invest at the lowest price and when to sell at it's highest.

The Stock Market hasn't made a profit across all it's shares in decades.
Possibly because you get idiot Presidents releasing stimulous packages on borrowed money and then allowing business to borrow. Then you see spikes in share pricing but often very short lived before they have to take account of the extra acquired debt.

Not anymore since the secret is out

only if they started off as multimillionaires

I don't believe so.

bitcoin

Can billionaires lose all of the money if the stock markets crashed

Only by fraud