How to only display post content on single page, not main page

I’m using the Simplest theme, which only has a main index, comments, and theme functions php. I would like to only display the post title on the main page and then on the single pages, the rest of the content. Since there is no single php folder for this theme I can’t outright delete the <?php the_content();?> code, without impacting the entire site.

Any suggestions? Do I need to create a single page php and import it to my child theme’s directory? Or is it as simple as entering some additional PHP code within the main index template? Here is a pastebin link to the current main index php: https://pastebin.com/YAtWCSpB