Hook to display element as product on category page

You have to override the shop loop to insert something between some products. The file where the loop happens is wp-plugins/woocommerce/templates/archive-product.php (this one). Override it using this guide and do something like this

$counter = 0;
while ( have_posts() ) {
  if($counter == 5){ //if you want your code between the 5th and the 6th product
    echo "<li ".wc_product_class( '', $product ).">";
    echo "My Html!";
    echo "</li>";    
  }
  the_post();

  do_action( 'woocommerce_shop_loop' );
  wc_get_template_part( 'content', 'product' );
  $counter++;
}

Please note that I took the <li> from the default woocommerce content-product.php template, you might want to look for the template in your theme, if it is overridden.

As you are trying to override a template through a plugin you need to inject your plugin woocommerce (or call it what you want) directory inside the woocommerce template search scope. This answer has a good code example on how to do it.