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How to list content entities in Drupal 9?

If you need to get a list of content entitles in your Drupal 9 website for example for drop down on the configuration page you can use the next code snippet
use Drupal\Core\Entity\ContentEntityType;

$content_entities = [];

// Get all entities definitions.
$entity_type_definations = \Drupal::entityTypeManager()->getDefinitions();

foreach ($entity_type_definations as $definition) {
  // Check that definition is instance of ContentEntityType
  if ($definition instanceof ContentEntityType) {
    $content_entities [] = $definition;