Change "Custom Text" field value
To extend and clarify Saud Alfadhli's answer for others (I'm not allowed to comment yet), hook_preprocess_views_view_field (hook_preprocess_views_view_fields with the "s" exists as well) works great for this. Both the view ID, display ID, and field ID can be easily inferred from this hook, and the field output can easily get overwritten (as both a string or render array). Here's a quick example replacing all occurrences of my second custom text field with a simple string:

/**
 * Implements hook_preprocess_views_view_field().
 */
function hook_preprocess_views_view_field(&amp;$variables) {
  if ($variables['view']-&gt;id() === 'my_view_id' &amp;&amp; $variables['field']-&gt;options['id'] === 'nothing_2') {
    $variables['output'] = 'Hi Mom!';
  }
}


Hope this helps!