Is exchanging a knight for a bishop and open the game a good strategy for beginners? I just learned that bishops are better in an opened game, but in a "normal" game is it good to follow this strategy?
No, it is not a good strategy (in any situation).

Each game is different and it is not a good idea to follow a strict strategy like this. Even in open positions there are situations where knights can become very strong. Take for instance the following position which is inspired by the Sicilian Najdorf (so would satisfy your semi-open requirement). The knight is very strong and cannot easily be removed from its dominant position. The black bishop on the other hand is stuck defending the pawn on d6 and does not have many options in general.

r2q1rk1/1p2bppp/p2p4/3Np3/4P3/8/PPP2PPP/R2Q1RK1 w - - 1 13


It is not possible to generalize that bishops are always stronger. You really need to analyze the position you have and base your strategy on this. A few things to look for:


badly placed pieces - which you could improve
weaknesses in the pawn structure - which could be attacked
open files - which you could occupy
more space - which you could take advantage of
better development - which you could take advantage of (typically by opening up the position)
insecure king - which you could attack


And of course you should analyze this for both sides in order to be aware of possible plans of your opponent.