How do I set the default values for creating a new content type?
For content or config entities you can set default values in a hook_ENTITY_TYPE_create():

use Drupal\node\Entity\NodeType;

/**
 * Implements hook_ENTITY_TYPE_create() for 'node_type'.
 */
function mymodule_node_type_create(NodeType $node_type) {
  $node_type-&gt;set('display_submitted', FALSE);
}