How to override content_type "Article"?
Use hook_entity_bundle_field_info()

Copying from the api docs

function mymodule_entity_bundle_field_info(\Drupal\Core\Entity\EntityTypeInterface $entity_type, $bundle, array $base_field_definitions) {
  // Add a property only to nodes of the 'article' bundle.
  if ($entity_type-&gt;id() == 'node' &amp;&amp; $bundle == 'article') {
    $fields = array();
    $fields['mymodule_text_more'] = BaseFieldDefinition::create('string')
      -&gt;setLabel(t('More text'))
      -&gt;setComputed(TRUE)
      -&gt;setClass('\Drupal\mymodule\EntityComputedMoreText');
    return $fields;
  }
}