Migrating Craft CMS 5 matrix entries to their own section

put-that-in-a-migration.php

<?php
/**
  * https://github.com/craftcms/cms/discussions/16774#discussioncomment-12357111
  *
  **/

use craft\elements\Entry;
use craft\helpers\Db;

$entries = Entry::find()->field('myMatrixField');
$section = Craft::$app->entries->getSectionByHandle('mySection');

foreach (Db::each($entries) as $entry) {
    Craft::$app->elements->duplicateElement($entry, [
        'fieldId' => null,
        'owner' => null,
        'primaryOwner' => null,
        'sectionId' => $section->id,
    ]);
}