where to find wp_tag_cloud ?

Hi

I am trying to change the appearance of the tag cloud on my blog. For this I am trying to find the function wp_tag_cloud in the wordpress files.

There is no explicit call to the tag cloud function from the theme I use, instead there is the following line in the sidebar.php file of the theme

<?php if ( !function_exists('dynamic_sidebar') || !dynamic_sidebar(1) ) : // begin primary sidebar widgets ?>

which I understand to be a fallback to default widgets if no customized sidebar is configured. So I assume the sidebar is build by the built-in functions of wordpress.
Although I searched for wp_tag_cloud through all files in the wordpress folder I did not find it, can anyone tell me where to find it?

Also a brief explanation how and with which files WP builds the dynamic sidebar would be terrific!

Thanks