You can create a custom CMS page layout file by using this below way.
File name should be like this :
cms_page_view_selectable_<CMS Page Identifier>_<Layout Update Name>.xml
=> For eg : If your cms page identifier is customhome, then file name should be cms_page_view_selectable_customhome_customhomelayout.xml
https://devdocs.magento.com/guides/v2.4/frontend-dev-guide/layouts/xml-manage.html
search :
For CMS Pages:
<?xml version="1.0"?>
<!--
/**
* Copyright © Magento, Inc. All rights reserved.
* See COPYING.txt for license details.
*/
-->
<page xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:noNamespaceSchemaLocation="urn:magento:framework:View/Layout/etc/page_configuration.xsd">
<head>
<!-- Add your css and js file tag here-->
</head>
<body>
<!-- Add your reference container, ref block etc. here -->
</body>
</page>