Where is PHP file for Product Description?

I have this set of PHP codes that displays metatags_keywords. For example, I put it:
includes\languages\english\html_includes\define_main_page.php
Now my main page displays metatags_keywords on the page (in addition to Meta section)

Now my question:
On each product page, below product description but before "Add to Cart", I need to insert this set of PHP codes to display metatags_keywords there. Which PHP file and which section should I insert these codes into? I am not a coder, so please point out the exact place. Don't wanna mess up anything.

I am using v1.3.9d
Thanks a lot in advance

Will they be the same for any product??

Yes, same for every product'a page. This is why I am looking for the PHP file for Product Description.

I am thinking it is probably:
includes\modules\product_listing.php

But I don't know where to insert.

On each product page
A copy for your template structure of

/includes/templates/template_default/templates/tpl_product_info_display.php

kobra:

A copy for your template structure of

/includes/templates/template_default/templates/tpl_product_info_display.php
thanks a lot. That's exactly what I need!!! :)