How to alter the content of a custom text field in a view in Drupal 8.7.

alter_custom_text_views_field.module

<?php

/**
 * @file
 * Primary module hooks for alter_custom_text_views_field module.
 *
 * To use in a module called "alter_custom_text_views", in Drupal 8.
 * 
 * @DCG
 * This file is no longer required in Drupal 8.
 * @see https://www.drupal.org/node/2217931
 */

use Drupal\views\Plugin\views\cache\CachePluginBase;
use Drupal\views\ViewExecutable;

/**
 * Prepare the custom field to be changed.
 *
 * Implements hook_views_pre_render();
 *
 * @see https://drupal8.ovh/en/tutoriels/238/alter-a-view-edit-view-result-programmatically
 * @see https://www.jeffgeerling.com/blog/2019/rendering-twig-templates-programmatically-drupal-8
 */
function alter_custom_text_views_field_views_pre_render(ViewExecutable $view) {
  if ($view->id() == 'view_machine_name' &&  $view->current_display == 'view_display_machine_name') {
    $customField = $view->field['field_machine_name'];
    // Don't render original template.
    $customField->options["alter"]["text"] = 'empty';
  }
}

/**
 * Alter custom text field content.
 *
 * Implements hook_views_post_render();
 */
function alter_custom_text_views_field_views_post_render(ViewExecutable $view, &$output, CachePluginBase $cache) {
  if ($view->id() == 'view_machine_name' &&  $view->current_display == 'view_display_machine_name') {
    // In this example I alter a leafleft display map.
    $points = &$output["#rows"]["#attached"]["drupalSettings"]["leaflet"]["leaflet-map-view-beacons-list-attachment-1"]["features"];
    foreach ($points as &$point) {
      // We load the template to render by myself.
      $markup = twig_render_template(drupal_get_path('module', 'alter_custom_text_views_field') . '/templates/pop-up-content.html.twig', [
        // Add here the twig variables to be enabled.
        'nid' => $point['entity_id'],
        'field_display_title_1' => 'field_display_title_1',
        'field_beacon_link' => 'field_beacon_link',
        // Needed to prevent notices when Twig debugging is enabled.
        'theme_hook_original' => 'not-applicable',
      ]);
      // Cast to string since twig_render_template returns a Markup object.
      $point['popup'] = (string) $markup;
    }
  }
}