XML Document "Flow"
There is one root node, commonly named either "ROOT" or "R", so you'll likely see xpaths like this:
/_R_/path/to/node
The root node name is customizable though, so your application or sites can use whatever you want.
Template Processing Order
This is a little confusing, but actually works well:
Sitemap loads a page specific template.
Page template loads another template usually called "main.xsl" or "html_main.xsl".
The main template then loads in all the other parts, including the outermost template, usually named "html_shell.xsl".
The outermost template is the one that initiates the template transformation process by matching the single root node.