It’s not a specific file. You add the code snippet to your theme’s functions.php file. For instance, the code bellow will put the Short description info instead of the default content of the “Additional information” tab. Be sure yo backup your file before modifying.
Yes, it’s not a specific file, besides it’s not related to our theme but to WooCommerce plugin because it’s default WooCommerce tab. If you want to change something use child theme, don’t make any changes in the parent theme because you will lose them after the update.