WP native tag suggest metabox. How does it process tag id?

As you know, there is a metabox that suggests tags as you type to input box. Is anyone knows how this function gets the tag’s ID (term_id) which is selected? I have looked at some of the core files but it seems only the tag name is retrieved and the ajax response returns only the name, no sign of id.

But it’s just impossible as the tag is updated by the tag’s ID but not it’s name upon submitting or updating the post

The core files i’ve looked at:

wp-admin/includes/meta-boxes.php
wp-admin/js/post.dev.js
wp-admin/admin-ajax.php
wp-includes/post.php
wp-includes/js/jquery/suggest.dev.js
wp-includes/taxonomy.php