Get a pretty good draft of our citation section

web/modules/custom/geo_citation/geo_citation.module

@@ -26,8 +26,72 @@ function geo_citation_entity_extra_field_info() {
 }
 
 /**
- * Implements hook_ENTITY_TYPE_view() for Drutopia articles.
+ * Implements hook_ENTITY_TYPE_view() for nodes.
  */
-function geo_citation_article_view(array &$build, \Drupal\Core\Entity\EntityInterface $entity, \Drupal\Core\Entity\Display\EntityViewDisplayInterface $display, $view_mode) {
+function geo_citation_node_view(array &$build, \Drupal\Core\Entity\EntityInterface $entity, \Drupal\Core\Entity\Display\EntityViewDisplayInterface $display, $view_mode) {
+  // This will only exist for the Drutopia article content type but don't think we need to check first.
+  if ($display->getComponent('geo_citation')) {
+    $build['geo_citation'] = [
+      '#markup' => geo_citation_pseudo_field($entity),
+      '#theme' => 'geo_citation_field',
+    ];
+  }
 
 }
+
+/**
+ * Implements hook_theme().
+ *
+ * We intentionally choose to do the citation in one big lump, because our
+ * expectation for making this more flexible would be to plug into bibcite or
+ * other citation API, rather than making it easy to re-theme this custom data.
+ *
+function geo_citation_theme($existing, $type, $theme, $path) {
+  return [
+    'geo_citation_field' => [
+      'variables' => [
+        'citation' => '',
+      ]
+    ]
+  ]
+}
+ * ...and it turns out giving theming capability to our pseudofield is optional.
+ */
+
+/**
+ * Assemble and return our citation as already-marked-up HTML.
+ */
+function geo_citation_pseudo_field($entity) {
+  $markup = '';
+  $article_type = $entity->field_article_type->getValue();
+  // If it's not a GEO Original article, there's nothing for us to do.
+  if (!isset($article_type[0]) || $article_type[0]['target_id'] != '303') {
+    return $markup;
+  }
+
+  $authors = $entity->field_authors->getValue();
+  $date = $entity->field_publication_date->value;
+  $year = substr($date, 0, 4);
+  $title = $entity->title->value;
+  $subtitle = $entity->field_subtitle->value;
+  $url = $entity->toUrl()->setAbsolute()->toString();
+
+  $markup .= '<div class="geo-citation">';
+
+  $tooltip = t("When citing this article, please use the following format:")
+  $markup .= '<h4><abbr title="' . $tooltip . '">' . t("Citations") . '</abbr></h4>';
+
+  $markup .= '<p>';
+  $markup .= "$authors ($year).&puncsp; $title";
+  if ($subtitle) {
+    $markup .= ":&puncsp; $subtitle";
+  }
+  $markup .= ".&puncsp; ";
+  $markup .= t("Grassroots Economic Organizing (GEO).");
+  $markup .= "&puncsp; $url";
+  $markup .= '</p>';
+
+  $markup .= '</div>';
+
+  return $markup;
+}