This page is rendered through a custom layout: _layouts/demo/simple.html, which imports a reusable Jet block from _layouts/demo/_blocks.html.

Copy this pattern — a _blocks.html defining {{ block name(args) }}...{{ end }}, a layout that {{ import "_blocks" }}s and {{ yield }}s it, and a note that points to the layout via layout: frontmatter — to build your own page designs.