For Original Fields please visit on your "woocommerce/templates/checkout/form-billing.php"
Here you will see a function
//This below line of code is responsible to get the fields
$fields = $checkout->get_checkout_fields( 'billing' );
//this below code is resposnible to echo those fields/labels etc
woocommerce_form_field( $key, $field, $checkout->get_value( $key ) );
If you want to go in depth please visit woocommerce/includes/wc-template-fucntions.php "function woocommerce_form_field( $key, $args, $value = null )", check this.
Надеюсь, это поможет вам!