Display selected categories onto post page

Question

If what I understand from your question is correct. You have a post that categorized in multiple categories and subcategories. I have already made the same in a website. use these following code bellow so a sub categories will displayed after the parent category.

//first  part
$allcats = get_the_category();
$parent_id = $all_ids = array();

foreach ($allcats as $cats) {
    //get all category id first
    $all_ids[] = $cats->term_id;

    // get top parent category
    if ($cats->category_parent === 0 ) {
        $parent_id[] =  $cats->term_id;
    }
}

//second part
$separator=" &raquo; ";
$term_ids = implode( ',' , $all_ids );

$hiearchy_cat="";
foreach ($parent_id as $parents) {
    $parent_name    = get_cat_name($parents);
    $parent_link    = get_category_link($parents);
    $parent_cat="<span class="parent"><a href="".$parent_link.'">'.$parent_name.'</a></span>';

    //get all sub category with certain ids
    $child_cats = wp_list_categories(
        array(
        'child_of' =>  $parents, 
        'title_li' => '',
        'style'    => 'none',
        'echo'     => false,
        'taxonomy' => 'category',
        'include'  => $term_ids,

        )
    );
    $child_cats = rtrim( trim( str_replace( '<br />',  $separator, $child_cats ) ), $separator );
    $hiearchy_cat .= '<b>'.$parent_cat.'</b> &raquo; '.$child_cats.'<br>';
}

//last output   
echo $hiearchy_cat;

Hope it helps