the pair is EUR/USD not USD/EUR.
you give the broker some money.
the broker says ok you gave me some money (in USD)I will let you use this much of my money to trade with (this is a margin account)
the broker says you can have the profit or loss I will cut you off before my money is at risk, you may lose all of yours
you tell the broker that you want to go long on the USD you do this by shorting the EUR/USD pair.
the broker will make the deal with the big banks this can happen a few different ways it depends on the brokers business model.
the broker is pays a spread on the deal when they make it. You pay a marked up spread or a commission to the broker. This is how the broker makes money. The broker has many clients some are long some are short at any one time the broker will try to maintain as much of a market neutral position as they can. Some will take the position against there client some will pass the lump of client trades on to the interbank market at sizes and times to try and maintain a neutral position.
the pair EUR/USD goes up on the chart the equity in your account at the broker goes down. the position you had them open is not worth as much now as when you had them open it. The broker will let you hold on to that position until its value is less then your account balance. This is an oversimplified margin call the exact level this happens at varies from broker to broker.
If EUR/USD goes down your position is worth more. You decide to close the position and make some profit. You click the button on your trading platform to tell the broker to close the position. The broker does this by selling USD and buying EUR. If the broker has done there job right they were market neutral the whole time they made there money on the spread they charged you it should not matter if you win or lose to the broker.
This all happens with the brokers money that they let you control. It does not mater what currency your account is based in.
On this gap open we see a bull trap set by bears trying to eat raccoons in the upward secondary tick pattern that is divergent to our Proprietary order flow indicator. You can see the x rated exhaustion level by which all of the sheep will be pulled in and sheared by the smart money