display all posts in current category

Based on this comment:

I would like to display all posts on the single-custom-post-type

You’re saying that, in the single CPT post view, you want to display both the single CPT post, and also a list of all CPT posts in the same category.

To do that:

1. Create single-{post-type}.php

2. The queried single CPT post is output via the default loop:

   if ( have_posts() ) : while ( have_posts() ) : the_post();
       // Loop markup here
   endwhile; endif
   

3. You will then need to retrieve the appropriate category:

   $cpt_cats = get_the_category();
   $cpt_cat = $cpt_cats[0]->term_id;
   

4. You will then need to add a secondary query to query all CPT posts in the given category

   $cpt_query = new WP_Query( array( 
       'post_type' => $cpt_slug, 'cat' => $cpt_cat, 'posts_per_page' => -1
   ) );
   

5. You will then need to create a custom loop to output your custom query:

   if ( $cpt_query->have_posts() ) : while ( $cpt_query->have_posts() ) : $cpt_query->the_post();
       // Secondary loop markup here
   endwhile; endif;
   // Don't forget this
   wp_reset_postdata();
   

(You will need to change the custom query parameters to suit your needs, and fill in the HTML markup where appropriate.)