You can't prevent every exchange and you don't have to: Just try to prevent you get two doubled pawns maybe.
What you can do is: (c4) g3, Bg2, Ne2, castling, then some time a3 and Nc3.
I don't understand your fascination for knights, but this is a way to avoid exchanges.
Knights are typically only superior to bishops in closed positions. If your opponent makes an unmotivated Bishop-for-Knight exchange, then try to avoid closing the position.
He will also fall slightly behind in Time by making the exchange... your Knight only moved once to reach c3 or f3, while his Bishop must move at least twice to exchange it, and those moves disappear from the board as the pieces come off... so this is another reason to steer for an open position after the B-for-N exchange: a Time advantage is more important in open positions.
you bring out your white knight to c3 or f3 and then they bring out a black bishop at g4 and b4. Then, they exchange, what would be the best response? because I want to keep my knight...