automatic login from, via 3rd party or deep link?

Here's what I managed to put together.
It's a controller for an "automated login" route - any auth requests directed to this URL will be redirected to ?destination, or a default page if ?destination is not given.
Any anon requests directed to this URL will be redirected to Open ID login via the ::authorize method of the Open ID provider - in this case "generic"

<?php

namespace Drupal\example_module\Controller;

use Drupal\Core\Controller\ControllerBase;
use Drupal\Core\Routing\TrustedRedirectResponse;
use Drupal\openid_connect\OpenIDConnectClaims;
use Drupal\openid_connect\Plugin\OpenIDConnectClientManager;
use Symfony\Component\DependencyInjection\ContainerInterface;
use Symfony\Component\HttpFoundation\RequestStack;

/**
 * Returns responses for auto-login route.
 */
final class AutoLoginController extends ControllerBase {

  /**
   * The controller constructor.
   */
  public function __construct(
    private readonly RequestStack $requestStack,
    private readonly OpenIDConnectClaims $claims,
    private readonly OpenIDConnectClientManager $pluginManager
  ) {}

  /**
   * {@inheritdoc}
   */
  public static function create(ContainerInterface $container): self {
    return new self(
      $container->get('request_stack'),
      $container->get('openid_connect.claims'),
      $container->get('plugin.manager.openid_connect_client')
    );
  }

  /**
   * Builds the response.
   */
  public function __invoke() {
    if ($this->currentUser()->isAuthenticated()) {
      $redirect = $this->requestStack->getCurrentRequest()->query->get('destination') ?? $this->getDefaultPage();
      return (new TrustedRedirectResponse($redirect))->addCacheableDependency($this->currentUser);
    }
    $this->saveDestination();

/*** CHANGE openid_connect.settings.generic TO AS APPROPRIATE FOR YOUR OPEN ID PLUGIN ****/
    $configuration = $this->config('openid_connect.settings.generic')
      ->get('settings');
/*****************************************************************************************/

    /** @var \Drupal\openid_connect\Plugin\OpenIDConnectClientInterface $client */
    $client = $this->pluginManager->createInstance(
      'generic',
      $configuration
    );
    $scopes = $this->claims->getScopes($client);
    $_SESSION['openid_connect_op'] = 'login';
    return $client->authorize($scopes);
  }

  /**
   * Helper.
   */
  protected function saveDestination() {
    // The destination could contain query parameters. Ensure that they are
    // preserved.
    $_SESSION['openid_connect_destination'] = [
      $this->requestStack->getCurrentRequest()->query->get('destination') ?? $this->getDefaultPage()
    ];
  }

  /**
   * Helper.
   */
  protected function getDefaultPage() {
    return '/example';
  }

}

Hope you have a second to answer this.

How do you get the openid_connect module to work with your controller? Besides the controller, what does one need to have in the example_module?

you need to hook this controller up to a route, e.g. via your example_module.routing.yml file

for my use case, i used the path 'login' like so:

example_module.auto_login:
  path: 'login'
  defaults:
    _title: 'Login'
    _controller: '\Drupal\example_module\Controller\AutoLoginController'
  requirements:
    _access: 'TRUE'

Then, for a "deep link", just prefix any path with login?destination=