Does it mean that they hold some money on those exchanges?
Otherwise they would have to transfer my coins to the destination exchange, make a trade and withdraw to my wallet, which would take quite some time and the rate could be significantly off from the original one.
P.S. Apologies for posting it to a wrong sub-forum.
Absolutely, they do have money ready to be traded off as soon as you make an order.
Delays which happen could be transaction related for example they could first have to wait for a confirmation of your payment and you would also have to wait for the sent crypto transaction to be confirmed in order to receive the digital assets in your wallet.
Best exchange rate is displayed at the time you execute an order and that the amount you get to receive whether after 10 minutes or 30 minutes