I guess I am missing something. How do you get your money to the exchange and how do you get it back? If you are buying 1 bitcoin for $830, how do you get you

I guess I am missing something. How do you get your money to the exchange and how do you get it back? If you are buying 1 bitcoin for $830, how do you get your $830 to the exchange? Do you give them your credit card number? Your paypal account? Don't they then have to pay the 2-3% to the credit card processor? And then I guess if you sell one bitcoin, how do they get your $825 to you? Do they send you a check? Do they wire it to you? Do they pay the costs for that transfer back to you? Or do you just assume you will keep the bitcoin and use it to buy something? And then how are we calculating the value of it vs. $? Do merchants use the same exchange rate or do they get to decide on the fly what exchange rate they are going to use? How do we know we are not losing at least 2% on that?