Issue #3468180 by john.oltman, sonfd, acbramley, smustgrave, tobiasb, nicxvan,...

    $block_content = $variables['elements']['content']['#block_content'] ?? NULL;

    if ($block_content instanceof BlockContentInterface) {

      $bundle = $content['#block_content']->bundle();

      $view_mode = strtr($variables['elements']['#configuration']['view_mode'], '.', '_');

      $view_mode = strtr($variables['elements']['content']['#view_mode'], '.', '_');

      $suggestions_new[] = 'block__block_content__view__' . $view_mode;

      $suggestions_new[] = 'block__block_content__type__' . $bundle;

      $suggestions_new[] = 'block__block_content__view_type__' . $bundle . '__' . $view_mode;

name: 'Block Content Theme Suggestions Test'

type: module

description: 'Support module for testing.'

package: Testing

version: VERSION

<?php

/**

 * @file

 * Support module for testing.

 */

declare(strict_types=1);

use Drupal\block_content\BlockContentInterface;

/**

 * Implements hook_preprocess_block().

 */

function block_content_theme_suggestions_test_preprocess_block(&$variables): void {

  $block_content = $variables['elements']['content']['#block_content'] ?? NULL;

  if ($block_content instanceof BlockContentInterface) {

    $variables['label'] = $block_content->label();

  }

}

<?php

declare(strict_types=1);

namespace Drupal\block_content_theme_suggestions_test\Hook;

use Drupal\Core\Entity\Display\EntityViewDisplayInterface;

use Drupal\Core\Entity\EntityInterface;

use Drupal\Core\Hook\Attribute\Hook;

use Drupal\Core\StringTranslation\StringTranslationTrait;

/**

 * Hook implementations for block_content_theme_suggestions_test.

 */

class BlockContentThemeSuggestionsTestHooks {

  use StringTranslationTrait;

  /**

   * Implements hook_entity_extra_field_info().

   */

  #[Hook('entity_extra_field_info')]

  public function entityExtraFieldInfo(): array {

    // Add an extra field to the test bundle.

    $extra['node']['bundle_with_extra_field']['display']['block_content_extra_field_test'] = [

      'label' => $this->t('Extra field'),

      'description' => $this->t('Extra field description'),

      'weight' => 0,

    ];

    return $extra;

  }

  /**

   * Implements hook_node_view().

   */

  #[Hook('node_view')]

  public function nodeView(array &$build, EntityInterface $entity, EntityViewDisplayInterface $display, string $view_mode): void {

    // Provide content for the extra field in the form of a content block.

    if ($display->getComponent('block_content_extra_field_test')) {

      $entity_type_manager = \Drupal::entityTypeManager();

      // Load a block content entity with a known UUID created by test setup.

      // @see \Drupal\Tests\block_content\Functional\BlockContentThemeSuggestionsTest::setUp()

      $block_content = $entity_type_manager->getStorage('block_content')->loadByProperties([

        'uuid' => 'b22c881a-bcfd-4d0c-a41d-3573327705df',

      ]);

      $block_content = reset($block_content);

      $build['block_content_extra_field_test'] = $entity_type_manager->getViewBuilder('block_content')->view($block_content);

    }

  }

  /**

   * Implements hook_theme().

   */

  #[Hook('theme')]

  public function theme(): array {

    // It is necessary to explicitly register the template via hook_theme()

    // because it is added via a module, not a theme.

    return [

      'block__block_content__view_type__basic__full' => [

        'base hook' => 'block',

      ],

    ];

  }

}

{{ label }}

<h1>I am a block content template for a specific bundle and view mode!</h1>