7. How do I execute or run an XML file?
You can't and you don't. XML itself is not a programming language, so normal XML documents don't ‘run’ or ‘execute’. XML is a markup specification language and XML files are just data: they sit there until you run a program which displays them (like a browser) or does some work with them (like a converter which writes the data in another format, or a database which reads the data), or modifies them (like an editor).
The water is muddied by XSL (both XSLT and XSL:FO) which use XML syntax to implement a declarative programming language. In these special cases you can ‘execute’ an XML file, by running a processing application like Saxon, which compiles the directives specified in XSLT files into Java bytecode to process XML.