From where the execution starts in Drupal
index.php inside Drupal root folder is the starting point of execution. It calls the bootstrap functions. Please see the code inside drupal 7 index.php file below.

define('DRUPAL_ROOT', getcwd());

require_once DRUPAL_ROOT . '/includes/bootstrap.inc';
drupal_bootstrap(DRUPAL_BOOTSTRAP_FULL);
menu_execute_active_handler();