They use whatever to convert all the currencies of the world into your base currency, usually USD/EUR. Then they transfer that to you, and you pay more conversion fees if that’s not your actual base.

I sell in USD, and then once a month they transfer my sales to me using Currency Cloud. At that point it gets converted at the mid-market rate + 0.25% IIRC. Since they don't actually make international transfers using banks, that amount is what actually arrives in my bank.