[ImpactBreaker]

Ayu can't really play instruments so obviously as a composer she can't arrange. (I guess she composes songs by humming them or making beeps on the cell phone i dunno ahaha). A simple practical way to know the extent of what the composer and the arranger does is just to keep on humming the melody of the song or listen to midis. How the lyrics will be sung, the pace, etc is mostly set by the composer. The composer will give the basis, the skeleton for the song to be constructed in it. The instruments choosen to be played, special effects like crashes, ringing bells, noises and other things will mostly be up to the arranger (though that can be also be part of the composer if it's from their interest to make crashes, collages and noises as part of the composition, a good example would be shiina ringo's compositions).