Arrangements, three different ways of working

We can agree or not about the details of an arrangement, about the use of some instruments in place of others, but I believe that everyone can agree on one aspect:

the arrangement is an essential phase for the correct outcome of any musical piece.

A great arrangement has the magical ability to turn a very ordinary melodic fragment into an excellent track.
Since this topic is of great importance, let’s try to understand exactly what we’re talking about.

What is a musical arrangement?