reddit's templating system is a bit less direct and explicit than some other systems. Here's a very high-level overview:

At some point in time, a class is constructed and .render() called on it. There are many of these classes, but what you'll find in common between them is that they inherit, directly or indirectly, from Templated. Here's an example:

class PasswordChangeEmail(Templated):
    """Notification e-mail that a user's password has changed."""
    pass

You'll note there is absolutely no body to the class; how then does it change functionality at all? The answer lies in some meta-magic in Templated's constructor, which stores self.__class__ as the render class.

Eventually we end up in tp_manager, and there we find that it uses the template name and style to identify the appropriate template file. The name is the aforementioned render class's name, lowercased; the style, if not passed in to .render(), defaults to html.

So then, when you see a line like this:

PasswordChangeEmail(user=user).render(style='email')

you should look for templates/passwordchangeemail.email. Simple enough once you know!

One more thing: since it's common to want to render the same data a number of ways (desktop-oriented HTML, mobile web, JSON, etc.), there's a mapping of extensions that force rendering to happen with a particular style. So if you tack on .compact to a url, it will render with whatever.compact instead of whatever.html, without you having to write code to switch based on that.