Quick tip – never, ever edit WP core files. Updates are released several times a year and you’ll lose your changes, among other reasons (such as security).
Your theme and the particular template file in question is what determines how WP search is processed. Assuming no search plugin is active, and your theme is calling get_search_form(), you can follow the logic path laid out in the WordPress Codex page for that function: https://codex.wordpress.org/Function_Reference/get_search_form
WordPress will look for a searchform.php file in the active theme. If it finds one, it loads it. If not, it falls back to the core search form. Therefore, the proper way to modify the WP core search form is to ensure your theme (or child theme) has a searchform.php file available.
In that, you can create a custom query to search only the values you wish to return results for.