Bishop are better on open position when not blocked by your own pawns.
Knight are better on closed position because they can- fork, and jump over each piece not something bishop or any other pieces can do.
How is better?
Well as you know when they are better now.
But let's make a scenario.
Your Bishop is not controlling much square (generally the more square your piece control the more active they are. And in the centre) and on other hand - your opponent knight. Can't get kick or is too active, you can trade it for that.
Also with knight if the position is too open and Bishop controlling lot of square you can trade.
But again depending on-how much piece you have, position matter, ect.
I hope this helps!
Who is the better ? knight or bishop ?