In fact, in general, composition and arrangement are complementary to create a work and somehow cover each other and make the work beautiful and pleasant. When a composer composes, his whole soul is involved in the text according to his knowledge of composing. Sometimes a composer composes without having a poem or song, and the poet or songwriter composes the melody of his poem or song accordingly. Which is not much different in either case.
The arranger is in charge of conducting the orchestra. And says what instruments should be used according to the atmosphere of the song and determines the style of work. And in the same genre, he implements his ideas and executes the notes related to that song with a certain elegance.
In fact, if the arranger does his job well, he can present a simple melody in the most beautiful way possible.