How did the first ever exchange onramp their fiat?

I know that satoshi never onramped his fiat, because he just mined the bitcoin and never offramped, but what if all the onramp exchanges dissappeared one day, would the only way to onramp be P2P ? If so, how did the first ever exchange onramp acquire crypto to permit people to onramp? like imagine if you opened the first ever exchange, a client comes to you to give you fiat, do you onramp that fiat by P2P with someone else who has crypto to send it to the client? How would P2P be efficient enough to handle multiple clients a day?