The general rules are:
Bishops and knights are technically worth the same. But the following things change this
In closed positions with a lot of pieces and pawns on the board, the knights are better.
In open positions and especially endgames with few pieces on the board, the bisop dominates the knight.
A pair of bishops is generally better than a pair of knights or a knight and a bishop. The bishop pair is a powerful tool as they can work together to control both colors.
Pretty much the power of the bishop or knight depends on the position. You are supposed to either build your position around the pieces you have or make your opponent's position worse by trading bishop for knight or vice-versa.