How to alter page content?
If you don't want to change the variables on field level you can parse the content in a #post_render callback after it is rendered:

/**
 * Implements hook_ENTITY_TYPE_view().
 */
function mymodule_node_view(array &amp;$build, EntityInterface $entity, EntityViewDisplayInterface $display, $view_mode) { 
  $build['#post_render'][] = function ($html, array $elements) {
    return str_replace('original', 'changed', $html);
  };
}