8 – Why is the region content already rendered as markup?

I’m attempting to craft some custom layout HTML in one region of my theme (uswds_base, but this doesn’t appear to be theme specific). The content of a typical region.html.twig file is like the following.

{% if content %}
  <div{{ attributes.addClass(classes) }}>
    {{ content }}
{% endif %}

Why is {{ content }} already rendered as a markup string in a region template? I’d like to be able to place individual blocks into different layout div’s. I know I can use {{ elements.block_1 }}, but that seems inefficient since the blocks have already been rendered.

I can’t figure out where the rendering is happening or if I’m just missing something obvious as to why {{ content }} isn’t an array.