That heading came from a wrapper layout. If you check out this page's source (tshirts), you'll notice some frontmatter applied between those --- blocks:

title:

The title we'll apply as the page heading and the browser tab title. You'll see this applied using {``} in the layout file described below 👇

layout:

The name of the layout file to apply. In this case, layout: layout translates to _includes/layout.njk since 11ty looks in the _includes folder by default.

You'll notice this page is wrapped in a layout. This pulls in a few parameters using 11ty's data cascade:

• title uses the "title" attribute from our page's front matter.
• content | safe renders our page.