How to create horizontal tabs programmatically in Drupal 8, requires Field Group module

Drupal8HorizontalTabs.php

<?php
// How to create horizontal tabs programmatically in Drupal 8, requires Field Group module
$form = array();
$form['my_field'] = array(
  '#type' => 'horizontal_tabs',
  '#tree' => TRUE,
  '#prefix' => '<div id="unique-wrapper">',
  '#suffix' => '</div>',
);
$items = array(
  array(
    'nid' => 1,
    'name' => 'Item 1'
  ),
  array(
    'nid' => 2,
    'name' => 'Item 2'
  ),
  array(
    'nid' => 3,
    'name' => 'Item 3'
  ),
  array(
    'nid' => 4,
    'name' => 'Item 4'
  ),
);
$counter = 1;
foreach ($items as $item) {
  $nid = $item['nid'];
  $form['my_field']['stuff'][$nid] = array(
    '#type' => 'details',
    '#title' => t('Item @no', array('@no' => $counter)),
    '#collapsible' => TRUE,
    '#collapsed' => TRUE,
  );
  $form['my_field']['stuff'][$nid]['form']['item_id'] = array(
    '#type' => 'value',
    '#value' => $item['nid'],
  );
  $form['my_field']['stuff'][$nid]['form']['item_name'] = array(
    '#type' => 'value',
    '#value' => $item['name'],
  );
  $form['my_field']['stuff'][$nid]['form']['remove'] = array(
    '#type' => 'checkbox',
    '#title' => t('Remove this item'),
    '#weight' => -51,
  );
  
  // more field definition
  $form['my_field']['stuff'][$nid]['form']['#tree'] = TRUE;
  $form['my_field']['stuff'][$nid]['form']['#parents'] = array('my_field', 'stuff', $nid, 'form');
  $counter++;
}

// under certain circumstances you have to attach the horizontal tabs library manually
$form['#attached']['library'][] = 'field_group/formatter.horizontal_tabs';
return $form;

Works on drupal 9 like a charm!

https://gist.github.com/leymannx/d99b43ffd867521fcabb4cd750fba998#file-drupal8horizontaltabs-php-L58 who is seeing only accordions using the snippet above and not tabs, change the library name to field_group/element.horizontal_tabs

super useful comment, thanks.