How do I render all posts of the same category in same layout?

I believe you are talking about single post pages. The template hierarchy does not make provision for single templates according to the category a post belongs to, so single-{$category}.php does not exist.

To make single-{$category}.php work, we can make use of the single_template filter

add_filter( 'single_template', function ( $template )
{
    global $post;
    // Check if our post has our specific category
    if ( !has_category( 1, $post ) ) // Change to your specific category
        return $template;

    // Locate and load our single-{$category}.php template
    $locate_template = locate_template( 'single-my_category.php' ); // Change to your exact template name
    if ( !$locate_template ) 
        return $template;

    // single-my_category.php exists, load it
    return $locate_template;
});

You can now just create our own custom single template and use it for any single post that belongs to the specific category you need to target