Mage2\Checkout\Controllers\CheckoutController::index PHP Method

    public function index()
    {
        $orderData = Session::get('order_data');
        if (Auth::check()) {
            $orderData['user_id'] = Auth::user()->id;
            Session::put('order_data', $orderData);
            return redirect()->route('checkout.step.shipping-address');
        }
        return view('checkout.index');
    }