Example 1. Taxonomy
Get list of all taxonomy terms
    $query = \Drupal::entityQuery('taxonomy_term');
    $tids = $query->execute();
Get term id of the vocabulary 'tags'
    $query = \Drupal::entityQuery('taxonomy_term');
    $query->condition('vid', 'tags');
    $tids = $query->execute();

Example 2. Node
Find all nodes where the title contains 'drupal'
    $query = \Drupal::entityQuery('node');
    $query->condition('title', '%drupal%','LIKE');
    $nids = $query->execute();

Example 3. User
Fiend all users who's name contains 'admin'
    $query = \Drupal::entityQuery('user');
    $query->condition('name', '%admin%','LIKE');
    $ids = $query->execute();

Example 4. Node with OR condition
    $query = \Drupal::entityQuery('node');
    $group = $query->orConditionGroup()
        ->condition('type', 'page')
        ->condition('type', 'article');
    $entity_ids = $query
        ->condition('title', '%drupal%','LIKE')
        ->condition($group)
        ->execute();

Example 5. With multiple AND + OR conditions
$query->condition('type', $type);
$query_or = $query->orConditionGroup();

$query_and = $query->andConditionGroup()
  ->condition('end1', $end1)
  ->condition('end2', $end2);
$query_or->condition($query_and);

$query_and = $query->andConditionGroup()
  ->condition('end1', $end2)
  ->condition('end2', $end1);
$query_or->condition($query_and);

$query->condition($query_or);

Example 6. Check A field value is NULL or is Not Null
$query = \Drupal::entityQuery('the_entity');
$query->notExists('field_to_check');//Check A field value is NULL
//OR
$query->exists('field_to_check');//Check A field value is NOT NULL
ids = $query->execute();

$current_user = \Drupal::currentUser();
$user_roles = $current_user->getRoles();
$current_uid = $current_user->id();
$user = User::load($current_uid);
$user->{'field_my_def_permission_group'}->value = 'xxx';
$user->save();