Let's imagine we want to send a user to custom route after login in our example to user edit form. How do we do it ?
We need to implement user_login hook
use Drupal\Core\Url;
use Symfony\Component\HttpFoundation\RedirectResponse;
use Drupal\user\Entity\User;
/**
* Implements hook_user_login().
*/
function mymodule_user_login(User $user) {
$url = Url::fromRoute('entity.user.edit_form', [
'user' => $user->id(),
]);
$redirect = new RedirectResponse($url->toString());
$redirect->send();
}