How to format shortcode’s HTML in external file

function wpse450_show_video( $atts ) {
    extract(shortcode_atts(array(
        'id'        => 0
    ), $atts ));

    if( is_numeric($id) ) {
        $ngvideo_title = get_the_title($id);
    }

    ob_start();
    include 'path/to/file/video.php';

    return ob_get_clean();
}

add_shortcode( 'ngvideo', 'wpse450_show_video' );

You can use $id, $atts in our file as you would in your function. The code in your file will behave just like it would have in your function with just one difference that you’ll have to echo the stuff instead of returning it like you would for your function