Drupal 8: Allow other modules to alter your data

allow_alter_your_date.php

<?php
// Allow other modules to alter your data.
$data = [
   '#type' => 'markup',
  '#markup' => t('This is the default text.'),
];
\Drupal::moduleHandler()->alter('MODULE_some_action', $data);

/**
 * Implements hook_MODULE_some_action_alter().
 */
function MODULE_MODULE_some_action1_alter(&$data) {
  // Here we have full access to $data array.
  $data['#markup'] = t('This was altered by MODULE.');
}