Source

File: wp-includes/link-template.php

function network_site_url( $path = '', $scheme = null ) {
	if ( ! is_multisite() ) {
		return site_url( $path, $scheme );
	}

	$current_network = get_network();

	if ( 'relative' === $scheme ) {
		$url = $current_network->path;
	} else {
		$url = set_url_scheme( 'http://' . $current_network->domain . $current_network->path, $scheme );
	}

	if ( $path && is_string( $path ) ) {
		$url .= ltrim( $path, '/' );
	}

	/**
	 * Filters the network site URL.
	 *
	 * @since 3.0.0
	 *
	 * @param string      $url    The complete network site URL including scheme and path.
	 * @param string      $path   Path relative to the network site URL. Blank string if
	 *                            no path is specified.
	 * @param string|null $scheme Scheme to give the URL context. Accepts 'http', 'https',
	 *                            'relative' or null.
	 */
	return apply_filters( 'network_site_url', $url, $path, $scheme );
}