Chords as a melody?

Hey, I was wondering something about the way you should write a melody down for an orchestra. Usually, when I give the melody to the instruments that are supposed to play them, I make them all play the same melody, in the sense that they do not form a chord together. So in the part that I added somewhere in this question, I have the flute, oboe and clarinet play the melody. They all start with the D instead of having all three instuments to play a chord together (For example flute: D, oboe: F, Clarinet: A = D minor chord).

now I know that a musician has the freedom of doing whatever he wants with his/her music, but I'd like to know if "advanced" musicians just write a melody, or write them as chords.

An example that just came in my mind is the beginning part of This is Halloween (there is a version on the internet arranged by James Kazik, which shows the sheet music), where the melody is G-G-G-G-G-Ab-F-F. The melody is played by the first violin, but the second violin and viola are playing along with 2 other notes that together make a chord.

I know that this is probably a bit of an odd question, but I hope it is somewhat comprehensible.