There's text under the pic explaining where to change/delete
the ad on the main page.

Have you searched the FAQS for info on the template override system? This chart will help you visualize where you may place
your template folders in the ZC file structure:
https://www.zen-cart.com/tutorials/i...hp?article=143

And this one:
https://www.zen-cart.com/tutorials/index.php?article=36

Your customized index.php file would go back to includes/languages/english/custom/index.php.