Get a list of categories that are related to posts

<!-- Display Categories Assigned to a post -->

                    <?php $taxonomy = 'category';

                    // Get the term IDs assigned to post.
                    $post_terms = wp_get_object_terms( $post->ID, $taxonomy, array( 'fields' => 'ids' ) );

                    // Separator between links.
                    $separator=",";
 if ( ! empty( $post_terms ) && ! is_wp_error( $post_terms ) ) {

                        $gterms = wp_list_categories( array(
                            'title_li' => '',
                            'style'    => 'none',
                            'echo'     => false,
                            'taxonomy' => $taxonomy,
                            'include'  => $term_ids,
                            'child_of' => 193
                        ) );

                        $gterms = rtrim( trim( str_replace( '<br />',  $separator, $gterms ) ), $separator );
                    }

put your parent cat id, for example, I have genre parent category, it has a lot of children, to display the children that are related to the current post you’re in put the id of the parent which is genre.
to output it plz use <?php echo $gterms; ?>

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