Twig: Parent template does not inherent what's ever inside the child template even using extends in the child
You aren't able to nest blocks in the template where that block is output - all content inside a block in the parent template is replaced by the child templates content, hence why you don't see your content block being output inside your head block. Likewise you can't use multiple extends within one template.

You can however nest blocks in a child template and output them in your extended template.

Base.twig (main template)

// Will display "content" and "sidebar content"
{% block content %}{% endblock %}

// Will only display "sidebar content"
{% block sidebar %}{% endblock %}


Child.twig

{% extends "Base.twig" %}

{% block content %}
  Content
  {% block sidebar %}
    Sidebar content
  {% endblock %}
{% endblock %}


More info on the topic can be found in Twigs documentation.