Yes they’ll be making off the spread, or more likely selling the trades to Citadel..doesn’t matter to me..I get the open when I trade a market order with TD Ameritrade, my fee free broker, and can’t expect anything better so what they do with my order is fine as long as I get the opening price and pay no fees…
By chance, I was looking at this just last week. It was the PDS for Airwallex that had me concerned. I believe money is held by Airwallex rather than at a bank ?