Each watchOS target includes an App and an Extension. App contains the UI stuff and Extension contains the actual logic (similar to Views and Models in MVC architecture in iOS).
Each WatchKit App has a
Interface.storyboard file which you design the app in it, and a
Assets.xcassets file to put your assets in.
Each WatchKit Extension has a
InterfaceController.swift file (actually a
WKInterfaceController subclass) which is similar to the
ViewController file in iOS.
To make a Hello World app:
Open up the
Locate the main
Apple Watch simulator will eventually show up with your app running.