FieldInstanceDefaults.php

<?php

namespace Drupal\field\Plugin\migrate\process\d7;

use Drupal\migrate\Attribute\MigrateProcess;
use Drupal\migrate\MigrateExecutableInterface;
use Drupal\migrate\ProcessPluginBase;
use Drupal\migrate\Row;
class FieldInstanceDefaults extends ProcessPluginBase {
    
    /**
     * {@inheritdoc}
     */
    public function transform($value, MigrateExecutableInterface $migrate_executable, Row $row, $destination_property) {
        [
            $default_value,
            $widget_settings,
        ] = $value;
        $widget_type = $widget_settings['type'];
        $default_value = $default_value ?: [];
        // In Drupal 7, the default value for email fields is stored in the key
        // 'email' while in Drupal 8 it is stored in the key 'value'.
        if ($widget_type == 'email_textfield' && $default_value) {
            $default_value[0]['value'] = $default_value[0]['email'];
            unset($default_value[0]['email']);
        }
        if ($widget_type == 'link_field' && $default_value) {
            $default_value[0]['uri'] = $default_value[0]['url'];
            $default_value[0]['options'] = [
                'attributes' => [],
            ];
            unset($default_value[0]['url']);
        }
        return $default_value;
    }

}