Source

File: wp-includes/sitemaps/class-wp-sitemaps.php

public function redirect_sitemapxml( $bypass, $query ) {
		// If a plugin has already utilized the pre_handle_404 function, return without action to avoid conflicts.
		if ( $bypass ) {
			return $bypass;
		}

		// 'pagename' is for most permalink types, name is for when the %postname% is used as a top-level field.
		if ( 'sitemap-xml' === $query->get( 'pagename' )
			|| 'sitemap-xml' === $query->get( 'name' )
		) {
			wp_safe_redirect( $this->index->get_index_url() );
			exit();
		}

		return $bypass;
	}