functions.php

<?php

if ( ! function_exists( 'cliff_primary_site_always_the_canonical' ) ) {
	/**
	 * Always have www.primary-site.com as the canonical URL.
	 *
	 * If site is loaded from www.example.com, for example.
	 *
	 * @link https://developer.wordpress.org/reference/functions/wp_get_canonical_url/
	 * @link https://kb.yoast.com/kb/canonical-urls-in-wordpress-seo/#code
	 *
	 * @param string $canonical_url
	 *
	 * @return string
	 */
	function cliff_primary_site_always_the_canonical( $canonical_url ) {
		// HTTP_HOST does not contain primary-site domain
		if ( false === strpos( $_SERVER['HTTP_HOST'], 'primary-site.com') ) {
			$canonical_url = str_replace( $_SERVER['HTTP_HOST'], 'www.primary-site.com', $canonical_url );
		}

		return $canonical_url;
	}
}
add_filter( 'get_canonical_url', 'cliff_primary_site_always_the_canonical' );
add_filter( 'wpseo_canonical', 'cliff_primary_site_always_the_canonical' );

if ( ! function_exists( 'cliff_example_add_to_cart_url' ) ) {
	/**
	 * If site is loaded from www.example.com, change the Add To Cart button link to something we confirmed works, like
	 * the Cart URL with the add-to-cart query param.
	 *
	 * @see \is_example()
	 * @see \WC_Product_Simple::add_to_cart_url()
	 * @see \vc_gitem_template_attribute_woocommerce_product_link()
	 * @see \wc_get_cart_url()
	 *
	 * @param string     $url       A URL string that already has 'add-to-cart' query param, if it should.
	 * @param WC_Product $the_class A class that extends this class, such as WC_Product_Simple.
	 *
	 * @return string
	 */
	function cliff_example_add_to_cart_url( $url, $the_class ) {
		if (
			is_example()
			&& $the_class->is_purchasable()
			&& $the_class->is_in_stock()
		) {
			$url = remove_query_arg( 'added-to-cart', add_query_arg( 'add-to-cart', $the_class->get_id(), wc_get_cart_url() ) );
		}

		return $url;
	}
}
// @TODO: The following (commented out) is likely not needed so try without it first!
// add_filter( 'woocommerce_product_add_to_cart_url', 'cliff_example_add_to_cart_url', 10, 2 );

if ( ! function_exists( 'cliff_example_home_page_to_landing_page' ) ) {
	/**
	 * Redirect example.com home page to its landing page.
	 *
	 * @see \is_example()
	 */
	function cliff_example_home_page_to_landing_page() {
		if (
			! is_front_page()
			|| ! is_example()
		) {
			return;
		}

		wp_redirect( 'https://www.example.com/some/deep-link/' );
		exit;
	}
}
add_action( 'template_redirect', 'cliff_example_home_page_to_landing_page' );

wp-config.php

<?php

if( ! function_exists( 'is_example' ) ) {
	/**
	 * Whether or not this is example.com (with or without WWW) request.
	 *
	 * @return bool
	 */
	function is_example() {
		if (
			! empty( $_SERVER['HTTP_HOST'] )
			&& (
				'example.com' === $_SERVER['HTTP_HOST']
				|| 'www.example.com' === $_SERVER['HTTP_HOST']
			)
		) {
			return true;
		} else {
			return false;
		}
	}
}

if ( is_example() ) {
	$example = 'https://www.example.com';
	define( 'WP_SITEURL', $example );
	define( 'WP_HOME', $example );
}