One bitcoin exchange isn't directly connected to another. So price in bitcoin can vary from one exchange to another. The price differences are not usually huge.
One bitcoin exchange is only weakly connected with another bitcoin exchange. If price in one exchange is higher than price at another, people can buy where price is cheaper, transfer bitcoin to another exchange and make money. This is possible because the nature of cryptocurrency is that you can transfer money around easily.
I couldn't imagine the same thing in forex exchange. Say price of USD is cheaper in roboforex than in forex.com, I wouldn't imagine people buying USD in robo forex, cashing it in a bank and wire or send the money to forex.com
So what actually happens in forex exchanges?
I know how money moves around in bitcoin exchanges. The user moves the money. If the users cannot easily move money around then the prices between exchanges will vary greatly.
In one exchange, coinexchange, for example, there was a time where ETH is in short supply. It took 10 hours confirmation to send ETH to coinexchange. So for a while, the price of ETH in coinexchange is 10 persent higher. Those who happen to have ETH in coinexchange can make a fortune selling ETH there. And then send the bitcoin to other exchange. Of course, they have to wait 9 hours for another cycle to work.
How do dollars and euro moves around in foreign exchanges? Do all exchanges have reserve accounts in various currencies in some banks? What happen if too many people buy EUR in one exchange and too many buy USD in another. What will happen? Will the bank automatically swift money around?