Getting kind of far afield, but that's exactly what the Android "Intent" framework is: any app can register handlers for any data/url. So your app now appears next to "youtube" when the user selects "share" on a video, etc...

It is part of iOS and has been since iOS 2.0. Every application developer gets to pick their own URL scheme and what any arguments passed to it do. This web site is just showing you the URL schemes for various applications.

You could not be more wrong. Not only is it part of iOS, it is part of UIApplication, the class on which any iOS app is built. Why even bother commenting, if you're just going to make stuff up?

I'm guessing you didn't even click on the web link, which, you will notice if you do, does not lead to any library. It's just a site (which I've never visited before this myself either... don't need to, to write or use inter-app URL launches) that hosts an informal registry of protocols along with information on what they launch.