Add content between blog post loop on main blog page

I’ll try to explain this as in depth as I can. I’m trying to modify the template page index.php, which is the page that lists all the blog posts, with a short summary. I’m trying to figure out where the code is that creates the blog post loop and where I need to add my content to get it added to that loop.

Here’s what I’m trying to do. I want to add AdSense ads between each of the blog posts on that page, so I need to find the code that’s creating and displaying that loop, so I can add the AdSense code there and have the ads appear between each blog post.

Thanks,
James