A way to determine where a URL is being generated?
In Drupal 6 many different things use the PathAuto module to quickly create URL Aliases for your content.

Automatic generation of aliases, the code that controls the administration and application of patterns, the database schema definitions &amp; more can be found within the module.

To answer your original question, it depends on what is creating that alias, a node or view or taxonomy page or whatever, but more times than not pathauto will be involved.

To see the automatic alias settings go to : admin/build/path/pathauto