There are 5 polos because if there were more, it would sound too crowded if you filled them all up, and I wanted to match with the colors. For your other questions, here are some reasons.
1: the colors are based on the kahoot options, Red for Beats, Green for Melodies, Yellow for Vocals, and Blue for Others.
2: try thinking of the normal polos hit boxes for the flying characters.
Edit: if you have some more questions, email me at @luna453008@gmail.com