Where would the cash come from?

Details are missing of course. I'm saying that there are many 20-50 person small businesses which are doing well for many decades.

Where do you get the money to buy them out to become a 20-50 person business?

True, but this company has taken on equity ( almost certainly with preferred shares ) investors. To buy them out, you would need cash. If you don’t have the cash, you need new investors. If the business is cash flowing and/or has assets, it may be possible to issue debt, effectively swapping equity investors for debt investors. The debt investor won’t care as much about growth as it does about you repaying your loan, but you are still stuck with an investor who has pre-negotiated rights ( and now the ability to liquidate the company if you don’t pay them ). Either way, you still have investors.

There are loads of 20-50 person small businesses who raised money in many different ways ( from personal savings to bank loans to customer prepays ). Many of those likely started with a small number of people, growing over many decades one paying customer at a time.