Material layouts have a selection of patterns that applications should follow for every screen they build. This sort of template is often called the scaffolding, and for mobile, it looks like this:
What is important about the scaffolding is that while it defines the basic layout of virtually every screen, it doesn't define how you should achieve this design, and even on Android, you'll find that there are several different ways of creating a screen with the preceding layout structure. Several elements are also optional: the Bottom Bar and floating action button are often left out because they aren't useful to a screen. The App Bar appears on most screens, but can be much larger and can also be folded away to provide the user with more reading space in the content area.
It's also important to understand that by default, the platform theming will put the App Bar (presented by the ActionBar class) into an Activity for you; it's also common to create your own App...
