How does extended hours trading work if the stock market is closed?

When a trader places a trade outside of the standard hours of major exchanges such as the New York Stock Exchange (NYSE) or the Nasdaq, the brokerage firm sends the order to one of its partner Electronic Communication Networks (ECNs).

ECNs are digital and automated and allow direct trading between parties without needing an exchange to mediate the transaction.

If there's a match for your order on the ECN (i.e., a sell order if you're looking to buy, or a buy order if you're looking to sell), your extended trade can be executed.