multisite-search.php

<?php

// ... this overrides the \SearchWP\Query instantiation found
// @link https://searchwp.com/documentation/search/supplemental-engine/
// e.g. $search_page is defined in the full snippet, this solely
// replaces the query to search across sites and return 'default' results.

$searchwp_query = new \SearchWP\Query( $search_query, [
	'engine' => 'supplemental', // The Engine name.
	'fields' => 'default',      // Retain site ID info with results.
	'site'   => 'all',          // Search all sites.
	'page'   => $search_page,
] );