What override do I need for that ?
Create an override for the articles layout.
Inside the html folder of the template, create a folder named "com_content", then another one "article" and copy there the article.php from
Then inside the copied article.php file, something like the code below would work. Place it where you want the button to display:
<div class="view-all-btn btn button">
<?php $catTitle = $this->escape($this->item->category_title); ?>
<?php echo '<a href="' . JRoute::_(ContentHelperRoute::getCategoryRoute($this->item->catslug)) . '" itemprop="genre" title="' . $catTitle . '">View All</a>'; ?>
Update / Answer to your comment:
Q: how do I control which categories this shows for?
Then you could connect the appearance of the view-all button with the settings of show category and link to category. Find inside the article.php the part of the code that is printing the category title and link and modify its html output to suit your needs, but keep its checking from the backend settings, so you can control from the admin when it will be shown.