How do I get all ContentEntities?
You can get around the error about the abstract method by using entity::loadMultiple, but you'll get a new error, because this only works from an entity subclass.

Instead of using the static method, you would have to do this dynamically:

First get all content entity types:

  $entity_types = \Drupal::entityTypeManager()-&gt;getDefinitions();
  $content_entity_types = array_filter($entity_types, function ($entity_type) {
    return $entity_type instanceof \Drupal\Core\Entity\ContentEntityTypeInterface;
  });
  $content_entity_type_ids = array_keys($content_entity_types);


Then load all entities for each type:

  $entities = [];
  foreach ($content_entity_type_ids as $type) {
    $entities[$type] = \Drupal::entityTypeManager()-&gt;getStorage($type)-&gt;loadMultiple();
  }