This is a site on the web.
It might be helpful for me to outline how this site is created as a resource to myself so I don't mess it up later.
Top-level pages - dir/index.html pages - take Front Matter to declare the title, description, and <body> class. All pages serve as their own components through the use of Nunjucks extends and variables.
Front matter can be in a separate json file of the same name, or at the top of the page.
Pick a layout to extend by placing {% extends 'layouts/your-layout.html' %} right under the front matter.
To set the HTML content of the page, place the structure inside a {% block %}. If the layout being extended is base.njk then it's {% block content %}{% endblock %}. If it's page.html then use {% block main %}{% endblock %}.
Sometimes, a post might require custom content not built into the layout. For example, my post on my 2020 migraines renders a heatmap using D3. The layout isn't built with the option to pass unique CSS and JS, or even extensive HTML. That's where the _includes/partials directory comes in.
Throw in a .html file with any styles, html, or js required to render a custom section of the post. In the markdown file of the post, use a Nunjucks {% include %} to find it and pass it in.