Drupal 8 get file or image field uri. Get default value if field is empty.

get_file_field_uri.php

<?php
use Drupal\file\Entity\File;

/**
 * Get the set or default image uri for a file image field (if either exist)
 *
 * @link https://drupal.stackexchange.com/questions/194176/loading-default-image-from-a-node-field-in-page-html-twig
 *
 * @param $entity \Drupal\Core\Entity\ContentEntityBase
 * @param $fieldName string
 * @return null|string
 */
function _get_file_field_uri($entity, $fieldName) {
  $image_uri = NULL;

  if( $entity->hasField($fieldName) ) {
    // Try loading from field values first.
    try {
      $field = $entity->{$fieldName};

      if ($field && $field->target_id) {
        $file = File::load($field->target_id);

        if ($file) {
          $image_uri = $file->getFileUri();
        }
      }
    }
    catch (\Exception $e) {
      \Drupal::logger('get_image_uri')->notice($e->getMessage(), []);
    }

    // If a set value above wasn't found, try the default image.
    if (is_null($image_uri)) {
      try {
        $field = $entity->get($fieldName);

        if ($field) {
          $default_image = $field->getSetting('default_image');

          if ($default_image && $default_image['uuid']) {
            $entity_repository = Drupal::service('entity.repository');

            /** @var $defaultImageFile File */
            $defaultImageFile = $entity_repository->loadEntityByUuid('file', $default_image['uuid']);

            if ($defaultImageFile) {
              $image_uri = $defaultImageFile->getFileUri();
            }
          }
        }
      }
      catch (\Exception $e) {
        \Drupal::logger('get_image_uri')->notice($e->getMessage(), []);
      }
    }
  }

  return $image_uri;
}

use-example.php

<?php

$uri = _get_file_field_uri($node, 'field_image');
$absolute_url = file_create_url($uri);

Excellent, thank you!