Creating customized php files in theme folder

Problem#1:

You can create a customized php file for your page using a Page Template

Page Templates

WordPress looks for template files in the following order:

1. Page Template — If the page has a custom template assigned, WordPress looks for that file and, if found, uses it.
2. page-{slug}.php — If no custom template has been assigned, WordPress looks for and uses a specialized template that contains the
   page’s slug.
3. page-{id}.php — If a specialized template that includes the page’s slug is not found, WordPress looks for and uses a specialized template
   named with the page’s ID.
4. page.php — If a specialized template that includes the page’s ID is not found, WordPress looks for and uses the theme’s default page
   template.
5. index.php — If no specific page templates are assigned or found, WordPress defaults back to using the theme’s index file to render
   pages.

In your scenario, your page template file would be page-order-page.php, wordpress would then load that file for when someone accesses (example.com/order-page).

Problem#2:

For problem 2, you would need to register query variables using the query_vars filter.

function wpse169050_register_query_vars( $vars ) {
    $vars[] = 'product_id';
    $vars[] = 'type';
    return $vars;
} 
add_filter( 'query_vars', 'wpse169050_register_query_vars' );

Your order url with the variables would then be

example.com/order-page/?product_id=001&type=digital

You can then get the value for both variables using get_query_var in your template file:

$product_id = get_query_var('product_id');
$type       = get_query_var('type');