How does Wordpress resolve permalinks internally?

Question

I am doing some deep modding of the rewrite system and custom post_types but before I can continue I must find out how WordPress handles the rewrites.

I found the function url_to_postid() but WordPress it self doesn't seam to use it to resolve permalinks into queries?

For example: If I hook into the pre_get_post functions, and browse a page or a custom post_type the provided query now include "is_page" or "attachment" etc but if i where to type a URL at random, this data will not show. Where does WordPress figure out if the URL is a page|posttype|attachment or not? And how can I hook into this?