why there is no google play store on iphone

Because the Google Play store has apps that will work on Android phones. They won't run on an iPhone. Many developers have apps with the same or very similar functionality on both the Google Play store and the App Store.

Google Play has absolutely NOTHING to do with Apple.

Talking about Google Play - I want to buy some stuff from there, but I don't want to waste any money, LOL. I've heard that I can buy stuff with some kind of "google play credits" or something.