How to find where media entity is used?

To find out where a media entity is used on your Drupal site, you can use the following query. Just replace the $media_id with your Media ID and execute the query.

$media_id = YOUR MEDIA ID;

// Get all media fields.
$media_fields = \Drupal::entityTypeManager()->getStorage('field_storage_config')->loadByProperties([
  'type' => 'entity_reference',
  'settings' => [
    'target_type' => 'media',
  ],
]);

// Get a query for each field table.
$queries = [];

foreach ($media_fields as $media_field) {
  $name = $media_field->getName();
  $entity_type = $media_field->get('entity_type');
  $table_name = "{$entity_type}__{$name}";
  $query = \Drupal::database()->select($table_name, 'T');
  $query->fields('T', ['entity_id', "{$name}_target_id"]);
  $query->where("{$name}_target_id = :id", [':id' => $media_id]);
  $result = $query->execute()->fetchAll();
  dsm($entity_type);
  dsm($result);
}

The easiest way to execute this query is probably in Devel PHP.