Where does overseas money from exports go?
There are two scenarios depending on the invoice currency of the trade.

If the trade is invoiced in USD, then the exporter receive USD from buyer, deposit this into the local bank. The local bank increase the amount in the exporter's bank account (in local currency), and put put this into central bank's reserve.

If the trade is invoiced in local currency, then the buyer has to exchange USD into local currency (similar process as above), then pay the exporter with local currency.