It doesn't use a tree of objects, so it's not exactly like that. egg:Paste#urlmap matches URL prefixes and redirects them to applications (which are in turn configured in another section or file). It doesn't use any kind of object publisher, so it doesn't construct any set of objects. Internally the application is free to parse the rest of the URL using an object publisher. I find the composition of distinct applications through assignment to be... well, it makes the applications no longer distinct. Monkeypatching objects to make the resolver happy is... well, not so hot either.
Typically an application will provide a simple callable, that in turn will instantiate the class. This can be provided by your framework, or embedded in the application itself. Since some frameworks don't have a root class, this concept isn't built in. (e.g., some frameworks simply have a root directory)