So who is responsible for Robinhood's orderbook? Are they partnering with an existing exchange to fulfill market/limit orders? Are they simply paying for the trading fees on behalf of users?

If Robinhood isn't posting their orderbook, why would anyone trust that there are no fees or commissions when they could bake their costs directly into the price? There's no market regulation (a la Reg NMS) for cryptocurrencies, so who's to say that the user isn't getting screwed on price and allowing Robinhood to take their cut?

But if they are partnering with an exchange, why pick one over the other? Who will pay for the withdrawal/transaction fees?