First you have to choose if you are going to copy Antimatter as a new theme and edit it, or if your going to extend Antimatter and use it as a parent theme. This is explained in the Theme docs, so I suggest reading over that.
Next, the logic for the modular pages is a combination of the page frontmatter (headers) and the Twig that provides the output logic.
You first would add a link in the modular headers:
title: Homepage Features
class: small
features:
- header: Markdown Syntax
icon: text-height
link: /some/link-a
- header: Twig Templating
icon: code
link: /some/link-b
- header: Smart Caching
icon: rocket
link: /some/link-c
...
Then you would modify the templates/modular/features.html.twig in the theme folder to add a hyperlink (feature.link) around the feature.header text:
<div class="modular-row features {{ page.header.class}}">
{{ content }}
<div class="feature-items">
{% for feature in page.header.features %}
<div class="feature">
{% if feature.icon %}
<i class="fa fa-fw fa-{{ feature.icon }}"></i>
<div class="feature-content icon-offset">
{% else %}
<div class="feature-content">
{% endif %}
{% if feature.header %}
<h4><a href="{{ feature.link }}">{{ feature.header }}</a></h4>
{% endif %}
{% if feature.text %}
<p>{{ feature.text }}</p>
{% endif %}
</div>
</div>
{% endfor %}
</div>
</div>
If you want to be able to add a link via the admin, you will need to modify the blueprints/modular/features.yaml file and add a text field for this new link element.