How Do I Get Thumbnails and Headlines?

I chose the Untitled theme because I liked the thumbnail and headline style shown here

https://theme.wordpress.com/themes/untitled/

But when I make a new post the whole post is displayed on my page.

https://thefinalmagazine.wordpress.com/

How do I get the thumbnails and headlines that you can click into to read the full article?

Sorry if its a silly question but I’m new to this.