Laravel - Use relationships between tables to get data from a table

I have an API with Laravel and I have decided to use relations between tables to make it work better, so I have related them using these functions in all migrations:

// 2020_05_26_185555_create_customers_table.php:27
$table->foreign('user_id')->references('id')->on('users');

And then I've implemented functions like the following (which get the records it's related to):

// app\User.php:80
public function customers()
{
    return $this->hasMany(Customer::class);
}

The problem is that from the Customers controller, I do not know how to use the function shown above to list all the clients of that user , that is, replace the following code:

// app\Http\Controllers\Customer\CustomerController.php:17
public function index()
{
    $user_id = Auth::id();
    $customers = Customer::where('user_id', $user_id)->get();
    return $this->showAll($customers, 200, $filters);
}

For something like the following (Just an example, it doesn't work, but it's the idea of ​​what I mean):

// app\Http\Controllers\Customer\CustomerController.php:17
public function index()
{
    $customers = User::customers(); // Ejemplo
    return $this->showAll($customers, 200, $filters);
}

Basically my problem is that I don't know how to use relations to get, for example, a user's clients , without using the conditions (which I currently do), because then relations don't make sense.

via Chebli Mohamed