As the name suggests, an XML-based layout is a specification of its widgets’
relationships to each other — and to containers — encoded in XML format.
Specifically, Android considers XML-based layouts to be resources, and as such
layout files are stored in the res/layout/ directory inside your Android project (or,
as we will see later, other layout resource sets, like res/layout-land/ for layouts to
use when the device is held in landscape)