When to use Class methods over Activities?

I'm getting introduced to Android programming and want to understand Activities a bit more (I'm coming in with basic-advanced java knowledge, basic knowledge in GUIs).

I currently assume that all methods will be contained in their respective activity, or in some helper class--whose methods wouldn't be shown to the user, (unless an activity used it), but whose methods could perhaps be used in multiple activities.

1. Are these assumptions correct?

2. When are helper classes even required?

3. Any good online resources regarding (specifically) classes vs activity use cases?

4. Any examples of a fairly simple, complete APK where I could look through and dissect the code, including XML? Hopefully something I could just import into Android Studio.

Thanks!