How do I output the value of a field from a Category selected from a Matrix?
Without knowing what your field handles are, this is the gist of how you’d do it:

{#
   Loop through each block within your Matrix field,
   assigning them to a `block` variable:
#}

{% for block in entry.myMatrixField %}

    {#
       Loop through each category within this Matrix block,
       assigning them to a `category` variable:
    #}

    {% for category in block.myCategoriesField %}

        {#
           Output the plain text field value:
        #}

        &lt;p&gt;{{ category.myPlainTextField }}&lt;/p&gt;

    {% endfor %}

{% endfor %}