But you are borrowing as many BTC as you have placed in your margin account.  That is why you can win on the price going up.  If you were not borrowing, you would just have been moving your BTC from your own wallet to 1Brokers.  If you place one BTC as margin and go long at leverage 1:1, then you borrow and additional BTC and can win or lose depending on what happens to the price.

If you were posting USD as margin and going long at 1:1, then you would not be borrowing but just buying.  Leverage where you post margin in the same currency as you trade is a bit strange, essentially the leverage is one higher than normal when going long, and one less than normal when going short.