#Question: why can '{{ content }}' be used in Jekyll templates?
that is because content is a special variable in all templates.
The content variable holds the page/post content including any sub-template content previously defined.
Render the content variable wherever you want into your template
Example Usage:

    <div id="sidebar"> ... </div>
    <div id="main">    {% raw %}
    {{ content }}{% endraw %}

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