Laravel Cashier Mollie integration

I am looking to implement Mollie via Cashier on Laravel 7. However, I can't understand how Mollie works.

I use the controller given in the documentation and I use the service https://webhook.site/ to receive callback from Mollie when making a payment. I get an answer from Mollie on the webhook URL with the transaction id.

SubscriptionController.php

<?php

namespace App\Http\Controllers;

use Laravel\Cashier\SubscriptionBuilder\RedirectToCheckoutResponse;
use Illuminate\Support\Facades\Auth;

class SubscriptionController extends Controller
{
    /**
     * @param string $plan
     * @return \Illuminate\Http\RedirectResponse
     */
    public function __invoke(string $plan)
    {
        $user = Auth::user();

        $name = 'premium';

        if(!$user->subscribed($name, $plan)) {

            $result = $user->newSubscription($name, $plan)->create();
            
            if(is_a($result, RedirectToCheckoutResponse::class)) {
                return $result; // Redirect to Mollie checkout
            }
            //create subscription
            return back()->with('status', 'Welcome to the ' . $plan . ' plan');
        }

        return back()->with('status', 'You are already on the ' . $plan . ' plan');
    }
    
    public function index()
    {
        return view('dashboard.upgrade', ['user' => Auth::user()]);
    }
            
}

My routes look like this:

http://localhost:8000/dashboard/subscribe/monthly

http://localhost:8000/dashboard/subscribe/annualy

Route::get('/dashboard/subscribe/{plan}', 'SubscriptionController')->name('userDashboardSuscribe');

What I can't understand is how Mollie works. What should I do when Mollie responds to the webhook URL?

This is more a question of integrating the solution rather than a technical question, however if you could accompany your answers with examples it would be nice.