Methods for creating, and updating multi-value fields in Drupal 8

Drupal: Add value to a multi field

Add a single value to an existing multi-value field
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$value = 'text';
$profile = Profile::load(uid);
$profile->get('field_name')->appendItem($value);



Add values to a multi-value during entity creation
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$node = $nodeManager->create([
  'type' => 'custom_node_type',
]);

$values = [
  'multi_value_0' => 421,
  'multi_value_1' => t('Sup Dawg'),
  'multi_value_2' => $something->id(),
];

$node->get('field_multivalue_name')->appendItem($values);

$node->save();