Hey Waad, flies are the flaps of cloth that cover pant zippers or buttons. Flies are not the actual zipper.

Hi Waad, Zikhona is technically correct but when referring to the zip/zipper on trousers the two are normally used interchangeably. I wouldn't expect most native speakers to know the difference in meaning between the two words.

"Fly" is also a verb, and an insect. But you probably already know this.

I've only heard "fly" when talking about trousers and pants. In fact, in Australia, I think it was the only word I used for this purpose when I was growing up. But we sometimes use "zipper" as well. On the other hand, "zipper" can be used when talking about trousers, bags, pencil cases, tents, etc.