How to query WP Multisite primary site link

Question

Use network_home_url( $path="", $scheme = null ).
You can see it in /wp-includes/link-template.php.

Examples:

// Root of main site
$network_home = network_home_url();

// About page on main site
$network_about = network_home_url( 'about/' );

In a network, the main site has always the same ID as the network ID. network_home_url() uses get_current_site() which return the global variable (sigh) $current_site, an instance of stdClass. This one is filled in wp-includes/ms-settings.php, which in turn is loaded in wp-settings.php before mu-plugins are loaded.

So you can rely on it in your plugin or theme.