Another theme example - preprocessing fields on a paragraph. Formats links with SVG icons next to the title value. FormattableMarkup creates a markup for each link which is being passed as a new variable into the template. This could also be done via hook_preprocess_field, but demonstrates that you can do this from other preprocess hooks.

mytheme.theme.php

<?php

/**
 * Implements hook_preprocess_paragraph().
 */
function mytheme_preprocess_paragraph(&$variables) {
  $paragraph = $variables['elements']['#paragraph'];
  $bundle = $paragraph->getType();

  if ($bundle == 'fancy_links') {
    $variables['fancy_links'] = NULL;

    if (!$paragraph->get('field_link')->isEmpty()) {
      $links = $paragraph->get('field_link')->getValue();

      foreach ($links as $link) {
        $external = UrlHelper::isExternal($link['uri']);

        // The two SVG icons are different. I removed them from the gist because they are rather large.
        
        if (!$external) {
          $url = Url::fromUri($link['uri'])->toString();
          $classes = 'hl__fancy-link';
          $icon = '<svg [attributes redacted for readability]"/>...</svg>';
        }
        else {
          $url = $link['uri'];
          $classes = 'hl__fancy-link hl__fancy-link--external';
          $icon = '<svg [attributes redacted for readability]"/>...</svg>';
        }

        $variables['fancy_links'][] = new FormattableMarkup('<a href="@url" class="' . $classes . '" title="@title"><span class="hl__button__text">@title</span><span class="hl__button__icon hl__button__icon--right">' . $icon . '</span></a>', [
          '@url' => $url,
          '@title' => $link['title'],
        ]);
      }
    }
  }
}