Craft Commerce add user to group on product purchase

addUserToGroupOnPurchase.php

   public function onAfterCompleteOrderHandler($event)
    {
        // @var Order $order
        $order = $event->sender;

        // LOGIC FOR SPECIFIC ITEM PURCHASES

        $newGroups = [];

        foreach ($order->lineItems as $lineItem) {

            $id = $lineItem->purchasable->product->id;

            if ($id == "10636") {
                self::log("Bought FOD eBook, add to group 7");
                $newGroups[] = 7;
            }
        }

        if (!empty($newGroups)) {

            // assignToGroups does not update the cached user model.
            // so instead we get the user from the db.
            $user = Craft::$app->users->getUserByUsernameOrEmail($order->email);

            if (!$user) {
                self::log("No user for $order->email found, can't add to groups...");
                return;
            }

            self::log("New Groups for order->email: $order->email user: $user->email found");
            self::log($newGroups);

            $groupIds = [];

            // Get existing group Ids
            foreach ($user->getGroups() as $group) {
                $groupIds[] = $group->id;
            }
            // Now, add the new group Ids
            foreach ($newGroups as $xValue) {
                $groupIds[] = $xValue;
            }

            $result = Craft::$app->users->assignUserToGroups($user->id, $groupIds);
            self::log("Added to groups is: " . ($result ? "true" : "false"));
        }
    }

Hi.
Thank you for this gist. I am just trying to implement this on my website.
So your code is a module I have to implement in my custom plugin. Correct ? :)

Yep, this code is part of a custom module (not a plugin in my case, but it could be either in a module or a plugin).