vendor\laravel\framework\src\Illuminate\Session\Middleware\StartSession.php

public function getSession(Request $request)
    {
        return tap($this->manager->driver(), function ($session) use ($request) {
            if ($request->cookies->get('PHPSESSID')) {
                $sessionId = $request->cookies->get('PHPSESSID');
            } else {
                $sessionId = $request->cookies->get($session->getName());
            }
            $session->setId($sessionId);
        });
    }