Well they are not really an exchange.
https://techcrunch.com/2018/01/25/free-cryptocurrency-trading-app/

says:

When you place a buy or sell order, Robinhood gives you an estimated price, connects to a slew of trading venues, exchanges, and market centers to find the lowest price, and uses its economies of scale to improve to score better prices over time. To counter market volatility, Robinhood puts a “collar” around your trade so if it can’t execute it at close to the estimated price, it waits for the price to return or lets you know.

Anyone knows what these exchanges might be? they are also showing prices for other currencies. Any ideas on how those prices are being derived.