How do we interpret a Drupal's URL?
If your looking at code, all the hook_menu function calls is how the non alias paths get created. Things like users and nodes use a % in the path creation; this turns into the argument for the page callback function. Read more about this here: Wildcard Loader Arguments.
node_menu()

function node_menu() {
...
  $items['node/%node'] = array(
    'title callback' =&gt; 'node_page_title', 
    'title arguments' =&gt; array(1), 
    'page callback' =&gt; 'node_page_view', 
    'page arguments' =&gt; array(1), 
    'access callback' =&gt; 'node_access', 
    'access arguments' =&gt; array('view', 1), 
    'type' =&gt; MENU_CALLBACK,
  );
...
}


user_menu()

function user_menu() {
...
  $items['user/%user_uid_optional'] = array(
    'title' =&gt; 'My account', 
    'title callback' =&gt; 'user_page_title', 
    'title arguments' =&gt; array(1), 
    'page callback' =&gt; 'user_view', 
    'page arguments' =&gt; array(1), 
    'access callback' =&gt; 'user_view_access', 
    'access arguments' =&gt; array(1), 
    'parent' =&gt; '', 
    'file' =&gt; 'user.pages.inc',
  );
...
}


The function that handles this menu information is menu_execute_active_handler().
Aliases happen as the 2nd to last part of the bootstrap in the drupal_init_path function. It converts /about-us into node/4 for internal use.

For more in depth information check out this link: Drupal menu system (Drupal 6.x)