In this case I got 1 BTC and i'll pay for 19100 USDT. The seller whose order I paid will receive 19000 USDT.
Where is 19100 -19000 = 100 USDT ? This is exchange profit or I wrong and exchange will not allow you to place such order?
Exchanges has nothing to do with you money that the transaction fee that is deducted. You set bitcoin to order limit to fill at $19100, once the price of bitcoin got to $19100 or higher is only when the order would be filled. People that set the price of bitcoin to sell at $19100 are the ones that you will by from as the order filled automatically.