How to find where Multisite::includeArea("contacts") comes from?

Question

1C-Bitrix

When you click on the submit button in the feedback form, no data is received. The form sends data to a file where there are no handlers, only require header.php and footer.php.
But is there a place to find the code for this module/infoblock?Multisite::includeArea("contacts")

<form action="/about/contacts/" method="POST" class="feedback_eshop_new">

Full code of /about/contacts/index.php file

<?require($_SERVER["DOCUMENT_ROOT"]."/bitrix/header.php");
$APPLICATION->SetTitle("Контакты");
$APPLICATION->SetPageProperty("title", "Контакты");?>

<? Multisite::includeArea("contacts");?>

<?require($_SERVER["DOCUMENT_ROOT"]."/bitrix/footer.php")?>

I did not find the description of the Multisite class in the Bitrix components .

Answer 1

Use reflection link

Answer 2

Use tracing, just throw an exception in this file

Answer 3

use a normal development environment (IDE), no need to write code in a notepad
in phpstorm for example:
right-click on the class name -> go to -> and a bunch of choices where, for example type declaration