wp-content/themes/twentytwentythree/templates/404.html

<!-- wp:template-part {"slug":"header","tagName":"header"} /-->

<!-- wp:group {"tagName":"main","layout":{"type":"constrained"}} -->
<main class="wp-block-group">
	<!-- wp:pattern {"slug":"twentytwentythree/hidden-404"} /-->
</main>
<!-- /wp:group -->

<!-- wp:template-part {"slug":"footer","tagName":"footer"} /-->