What is the difference between the_content and content.php?
the_content is a template tag that displays the post content entered into the WYSIWYG editor in the admin.
You can use get_template_part (); to load a template partial like content.php into a template, like index.php. Because a partial is a reusable piece, you can call it in lots of different templates. This saves you from repeating a bunch of code unnecessarily and it makes updates much easier!
So now you know that you can call 'the_content();' inside your content.php template partial and be perfectly content about it! Haha!