Seems like you are doing everything right. Make sure that in /catalog/view/theme/YOURTHEME/template/product/category.twig YOURTHEME is actually your current theme. From the tutorial for thumb and description take this code
{% if thumb or description %}
<div class="row"> {% if thumb %}
<div class="col-sm-2"><img src="{{ thumb }}" alt="{{ heading_title }}" title="{{ heading_title }}" class="img-thumbnail" /></div>
{% endif %}
{% if description %}
<div class="col-sm-10">{{ description }}</div>
{% endif %}
</div>
<hr>
{% endif %}
Or only for description take
{% if description %}
<div class="col-sm-10">{{ description }}</div>
{% endif %}
And replace it anywhere in this template file (almost everywhere actually).
The most important is to clear twig cache. On howto do that i have described the process in the other article earlier, please read
https://stackoverflow.com/a/61524855/3187127
After that - click Ctrl F5 if you are using Chrome or Firefox (or other combination in your browser which reloads page with cache cleaning) while you are on a category page.