Android :: Not Understanding A Basic Concept Of Application Development

First and foremost, are there many android developers here? Is this a good place for Android related discussions?I seem to be missing a rather large concept of Android development. The gist is I am struggling understanding how to tie an application together. I'm not sure how to explain it, so I thought I would do my best with an example from the Android ApiDemo... assuming you are familiar with it.

Inside the com.example.android.apis.view namespace of the ApiDemo, there is a class called Animation3.java.Animation3 inherits the activity class and there is some code inside to display animation.I can't find a reference to the class (Animation3) anywhere in the demo code (except for its definition obviously). The only mention I found is in the manifest xml file. So how the heck does this activity get started? Don't we need to create an instance of the class somewhere and fire off a method to start it? I don't understand how to generate the code that ultimately glues this class to the rest of the application.Additionally, what about other classes like views or viewgroups? How do I generate code outside the class that initiates/starts/uses/calls (insert proper term) the class.I would appreciate any code examples as well as any concept explanation or reference documents. So far I've read pages and pages on activities and views but I'm really struggling how to tie things together.