Where to paste the code to apply it throughout the site?

Question

opencart

Hello! I found on the Internet how to hide breadcrumbs in the product card in the mobile version.

There he is:

if(preg_match("/(android|avantgo|blackberry|bolt|boost|cricket|docomo|fone|hiptop|mini|mobi|palm|phone|pie|tablet|up\.browser|up\.link|webos|wos)/i", $_SERVER["HTTP_USER_AGENT"])){
           unset($data['breadcrumbs']);
}

And I thought that, instead of hiding, through else, you can show messages on the mobile version and other messages on the PC.
Everything worked out, I inserted this code into the "product" controller and everything worked out, but where to insert it to show in the right places without adding it to each controller. Or is it not possible?
In fact, I need to display messages in mobile in the product card, categories and on the main page.

But the knowledge was only enough for the goods, and then, according to someone else's method ...
Please help me solve the problem

Answer 1

If you need to add something on all pages, then work in the controller:
common/header
or
common/footer