Drupal 8 pass data down ER fields

example.module.php

<?php

/**
 * @file
 * An example of how to pass your custom variable to the first item in a multi-value
 * entityreference field.
 */
 
/**
 * Implements hook_preprocess_field().
 * Change `field_name` to your field machine name.
 */
function example_preprocess_field__field_name(&$variables) {
  $variables['items'][0]['content']['#my_custom_var'] = 'foobar!';
}

/**
 * Implements hook_preprocess_node().
 */
function example_preprocess_node(&$variables) {
  if (isset($variables['elements']['#my_custom_var'])) {
    $variables['foo'] = $variables['elements']['#my_custom_var'];
  }
}

node.html.twig

{#
// .. usual template stuff
#}
<article>
{% if my_custom_var %}
  Got {{ my_custom_var }}!
{% else %}
  {{ content }}
{% endif %}
</article>