Where does the main page get its content from?

I am totally new to Zola and I am trying to figure out where does the content of the main index page come from? Is it from the templates/index.html or can I provide it as a markdown file in the contents/ folder?

You can provide content for the homepage in content/_index.md

Thanks. I created that file, but the content does not show up on the main page. Is it something to do with my templates or some configuration option?

$ cat content/_index.md 
title = "Main"
date = 2019-11-27

Main page from _index.md
$ cat templates/index.html 
{% extends "base.html" %}

{% block content %}
<h1 class="title">
Check out the <a href="/main">main page</a>
{% endblock content %}

Are you using a theme? The template you’ve shown is not showing any content of the page, you will need something like {{section.content | safe }} somewhere to display the text

No I have not got to the point of themes yet. This was it. Thank you!