Jekyll: List all categories with according post count and show and link all post items listed in the according category

list-categories-count.liquid

<h2>Categories</h2>
<ul>
{% assign categories_list = site.categories %}
  {% if categories_list.first[0] == null %}
    {% for category in categories_list %}
      <li><a href="#{{ category | downcase | downcase | url_escape | strip | replace: ' ', '-' }}">{{ category | camelcase }} ({{ site.tags[category].size }})</a></li>
    {% endfor %}
  {% else %}
    {% for category in categories_list %}
      <li><a href="#{{ category[0] | downcase | url_escape | strip | replace: ' ', '-' }}">{{ category[0] | camelcase }} ({{ category[1].size }})</a></li>
    {% endfor %}
  {% endif %}
{% assign categories_list = nil %}
</ul>

{% for category in site.categories %}
  <h3 id="{{ category[0] | downcase | url_escape | strip | replace: ' ', '-' }}">{{ category[0] | camelcase }}</h3>
  <ul>
    {% assign pages_list = category[1] %}
    {% for post in pages_list %}
      {% if post.title != null %}
      {% if group == null or group == post.group %}
      <li><a href="{{ site.url }}{{ post.url }}">{{ post.title }} <time datetime="{{ post.date | date_to_xmlschema }}" itemprop="datePublished">{{ post.date | date: "%B %d, %Y" }}</time></a></li>
      {% endif %}
      {% endif %}
    {% endfor %}
    {% assign pages_list = nil %}
    {% assign group = nil %}
  </ul>
{% endfor %}

when i click the category, my posts don't show up. do I have to set the permalink and create a page?
because now it just points to category/year/month/day/category

Let's say I don't use a default front-matter variable like tags or categories, but instead a non-default one like author / authors in each post? I does work for categories, I have these too, but I want it for authors too.
If i replace site.categories with site.authors I get a Liquid Exception: 0 is not a symbol nor a string in xxx.html