<?php

use TYPO3\CMS\Core\Utility\GeneralUtility;


function generateMenu($id)
{
    $PageRepository = GeneralUtility::makeInstance(\TYPO3\CMS\Core\Domain\Repository\PageRepository::class);
    $subPage = $PageRepository->getMenu($id);
    return $subPage;
}

generateMenu(1);

<?php

function setPageSlug($uid) {

    /*
    tablename => name of the table with the slug field
    slug_field_name => name of the corresponding slug field
     */

    $tablename = "pages";
    $slug_field_name = "slug";

    $fieldConfig = $GLOBALS['TCA'][$tablename]['columns'][$slug_field_name]['config'];
    $slugHelper = \TYPO3\CMS\Core\Utility\GeneralUtility::makeInstance(\TYPO3\CMS\Core\DataHandling\SlugHelper::class, $tablename, $slug_field_name, $fieldConfig);

    $connection = \TYPO3\CMS\Core\Utility\GeneralUtility::makeInstance(\TYPO3\CMS\Core\Database\ConnectionPool::class)->getConnectionForTable($tablename);
    $queryBuilder = $connection->createQueryBuilder();

    $queryBuilder->getRestrictions()->removeAll()->add(\TYPO3\CMS\Core\Utility\GeneralUtility::makeInstance(\TYPO3\CMS\Core\Database\Query\Restriction\DeletedRestriction::class));
    $statement = $queryBuilder->select('*')->from($tablename)->where(
        $queryBuilder->expr()->eq('uid', $uid)
    )->execute();

    $record = $statement->fetch();

    $slug = $slugHelper->generate($record, $record['pid']);

    // Update
    $queryBuilder = $connection->createQueryBuilder();
    $queryBuilder->update($tablename)->where(
        $queryBuilder->expr()->eq('uid', $uid)
    )->set($slug_field_name, $slug)->execute();

    //var_dump($slug);
    return $slug;
}




$queryBuilder = \TYPO3\CMS\Core\Utility\GeneralUtility::makeInstance(\TYPO3\CMS\Core\Database\ConnectionPool::class)->getQueryBuilderForTable('pages');
$statement = $queryBuilder
    ->select('uid', 'slug')
    ->from('pages')
    ->execute();
while ($row = $statement->fetch()) {
    \TYPO3\CMS\Extbase\Utility\DebuggerUtility::var_dump($row);
    setPageSlug($row["uid"]);
}