themes-wordpress/assembler/templates/404.html

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

<!-- wp:group {"tagName":"main","metadata":{"name":"Main"},"layout":{"type":"constrained","contentSize":"400px","wideSize":"720px"}} -->
<main class="wp-block-group">
    <!-- wp:pattern {"slug":"assembler/hidden-404"} /-->
</main>
<!-- /wp:group -->

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