Field

On drupal 8 every elements (almost) are an entity, as any entity you can render a node.

Note : The tutorial is usins States VISIBLE, but can also use for other html parameters such as : REQUIRED, EXPENDED, CHECKED ...

The selector is an "JQUERY_SELECTOR"

Example.
Step 1 : Create field storage.
\Drupal\field\Entity\FieldStorageConfig::create(array(
  'field_name' => 'field_text',
  'entity_type' => 'node',
  'type' => 'text',
  'cardinality' => -1,
))->save();