What does an exchange do when selling/buying crypto?

[Bitcoin]

What does an exchange do when selling/buying crypto?

I know well that I can sell and buy cryptocurrencies like BTC and ETH on exchanges like Coinbase, I do it almost every week.

But my question is: how does an exchange materially convert dollars or euros to bitcoins? If I were the exchange and a user gives me one hundred dollars and tells me to "convert them to bitcoins"; what is it that I as the exchange then do?

Source: What does an exchange do when selling/buying crypto?