This ends up as a kind of family-tree structure in the application's memory (tree structures are a common way for programs to store related data).
There are languages developed specifically to express XML schemas.
The Document Type Definition (DTD) language, which is native to the XML specification, is a schema language that is of relatively limited capability, but that also has other uses in XML aside from the expression of schemas.
Two more expressive XML schema languages in widespread use are XML Schema (with a capital S) and RELAX NG.
It contains a reference to an XSD file, which you will also need to copy from below or download and enter into the webpage (in a second step, when you are prompted for it) to see the validation results.
The Android operating system can be divided into the four areas as depicted in the following graphic.