Customize in category page

Review the template hierarchy from the Codex to gain a better understanding of what files are read when. This will help you to understand what’s called in what situation (tag/category/front-page/etc).

Content.php is never mentioned in the documentation as its not a part of the template hierarchy. Theme developers will use different file names and will reference them within the loop.

Most of the themes by Automattic use this technique to call the various parts to build out the content. Review your index.php, page.php, or post.php to see if content.php is used there.