How to decrement product quantity after checkout in Laravel (With Session)

Can you help me how to decrement after doing a checkout. So far this is the only idea I have but it shows an error called "Object of class Illuminate\Database\Eloquent\Collection could not be converted to number ". Thank you for the reply. Also I am using the one with session.

Note. I edit and put another image for more clearer explanation and here is the code

public function postCheckout(Request $request){
        $books = Product::all();
        if (!Session::has('cart')){
             return view('shop.shoppingcart');

        }
        $oldCart = Session::get('cart');
        $cart = new Cart($oldCart);
        $order = new Order();
        $order->cart = serialize($cart);
        $order->address = $request->input('address');
        $order->name = $request->input('name');
        
        Auth::user()->orders()->save($order);
        
        $q=$cart->totalQty;
        $r=$books-$q;
      Product::table('books')
      ->where('id',$id) 
    
      ->update(array('quantity' => $r));
        Session::forget('cart');

        return redirect("home");
    }

And the error is "Object of class Illuminate\Database\Eloquent\Collection could not be converted to number"

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