Some third party apps that involved adding information or code to the collections page or related items often ask you to open a file called product-grid-item.liquid or collection.liquid.
In Out of the Sandbox themes, most of the time you will want to open a file called product-thumbnail.liquid and is located in the Snippets folder. This file typically has the code you're looking for when trying to modify the appearance or functionality of how products are shown on collection pages or related items.
If you are attempting to install the Shopify Product Reviews app and are using a more recent version of our themes, the Shopify Product Reviews App is already integrated. To see if your version is compatible, go under Sales Channels > Online Store > Themes > Customize theme (indigo button) > General settings > Product grid > Reviews. If you don't see this panel, you should consider upgrading.