Google has quite a task ... to manage an installed base of over a thousand different types of Android devices. Multiple sizes, shapes, features, etc. The positive way to describe this is variety. On the other hand, it has also been refered to as fragmentation.
So how do they do it? In short, leverage. A good metaphor is a mechanical shovel. Relatively compact machines can move and lift very heavy objects using a system of joints and lever articulation. Android does something similar.
The code in Android applications doesn't manipulate devices directly. There are intermediate joints and levers that make the task manageable. Application code is programmed using component names to refer to layout elements. Layouts are adjusted to device characteristics automatically by the Android operating system. This lets an individual line of code manipulate many different types of devices.
This is a simplified view of how it all happens. There are a number of Android features that make it work: Themes, Styles, Action Bars, Fragments, Resources and much more. When implemented properly, an Android application can move a thousand different types of devices.