That doesn't seem correct to me. Using Rivian as an example, at what point did any founders become "billionaires" i.e. their stock/ownership value was at a bill

That doesn't seem correct to me. Using Rivian as an example, at what point did any founders become "billionaires" i.e. their stock/ownership value was at a billion? I'm guessing long before they had a working assembly line. I think people misunderstand how little a billion dollars can be when it refers to net worth and particularly one's stock ownership.