Dependency Injection for Accessing Services in Drupal 9

use Drupal\Core\Entity\EntityTypeManagerInterface;
use Symfony\Component\DependencyInjection\ContainerInterface;
use Drupal\Core\Config\ConfigFactoryInterface;
use Drupal\Core\Entity\EntityTypeBundleInfo;
use Drupal\Core\Entity\EntityTypeBundleInfoInterface;

Inside your class where you want to inject the Drupal 9 services:

/**
   * Get access to entity type manager.
   *
   * @var \Drupal\Core\Entity\EntityTypeManagerInterface
   */
  protected $entityTypeManager;

  /**
   * The config factory.
   *
   * @var \Drupal\Core\Config\ConfigFactoryInterface
   */
  protected $configFactory;

  /**
   * Get access to Bundle Info.
   *
   * @var \Drupal\Core\Entity\EntityTypeBundleInfoInterface
   */
  protected $entityTypeBundleInfo;

  /**
   * Class constructor.
   */
  public function __construct(EntityTypeManagerInterface $entity_type_manager,
    ConfigFactoryInterface $config_factory, EntityTypeBundleInfoInterface $entity_type_bundle_info) {
    $this->entityTypeManager = $entity_type_manager;
    $this->configFactory = $config_factory;
    $this->entityTypeBundleInfo = $entity_type_bundle_info;
  }

  /**
   * {@inheritdoc}
   */
  public static function create(ContainerInterface $container) {
    // Instantiates this form class.
    return new static(
      // Load the service required to construct this class.
      $container->get('entity_type.manager'),
      $container->get('config.factory'),
      $container->get('entity_type.bundle.info')
    );
  }

<hr />

Brain storming:
// dump($this->entityTypeManager->getStorage($entity_type)->loadMultiple());
      // return bundles from entityTypeManager
      // $bundle_info = $this->entityTypeManager->getStorage('field_config')->load('node.' . $entity_type . '.body')->getBundleInfo();

      // $bundles = $this->entityTypeManager->getStorage('entity_type')->load($entity_type)->getBundleEntityType();
      $bundles_data = $this->entityTypeManager->getStorage($entity_type)->loadMultiple();

      $bundles = $this->entityTypeBundleInfo->getBundleInfo($entity_type);

      // kint($bnd);

      // kint($this->entityTypeManager->getDefinition($entity_type)->getBundleEntityType());

      // kint($this->entityTypeManager->getStorage('entity_type'));
      // $this->entityTypeManager

      // $bnd = $this->entityTypeManager->get('entity_type.bundle.info')->getBundleInfo($entity_type);
      // $bnd = array_keys($bnd);

      // foreach($bundles_data as $b => $bf) {
      //   $bundles[] = $bf->bundle();
      // }

      // echo '<hr><hr>';
      // // kint($bundles);
      // foreach ($bundles as $bundle) {
      //   // kint($bundle);
      //   echo '..................';
      // }
      // echo '<hr><hr>';

      // echo '<hr />';

      // $t = $this->entityTypeManager->getStorage('taxonomy_term')->load(2);
      // echo $t->bundle().'<br /><br /><br /><br /><br /><br />';

      // $bundles = [];