<?php
/**
 * Copyright © Magento, Inc. All rights reserved.
 * See COPYING.txt for license details.
 */
declare(strict_types=1);

use Magento\Quote\Api\CartRepositoryInterface;
use Magento\Quote\Model\QuoteFactory;
use Magento\Quote\Model\ResourceModel\Quote as QuoteResource;
use Magento\TestFramework\Helper\Bootstrap;

/** @var QuoteFactory $quoteFactory */
$quoteFactory = Bootstrap::getObjectManager()->get(QuoteFactory::class);
/** @var CartRepositoryInterface  $cartRepository */
$cartRepository = Bootstrap::getObjectManager()->get(CartRepositoryInterface::class);
/** @var QuoteResource $quoteResource */
$quoteResource = Bootstrap::getObjectManager()->get(QuoteResource::class);

$quote = $quoteFactory->create();
$quoteResource->load($quote, 'test_quote', 'reserved_order_id');

$quote->setCustomerEmail('guest@example.com');
$cartRepository->save($quote);