Source

File: wp-includes/functions.php

function get_main_network_id() {
	if ( ! is_multisite() ) {
		return 1;
	}

	$current_network = get_network();

	if ( defined( 'PRIMARY_NETWORK_ID' ) ) {
		$main_network_id = PRIMARY_NETWORK_ID;
	} elseif ( isset( $current_network->id ) && 1 === (int) $current_network->id ) {
		// If the current network has an ID of 1, assume it is the main network.
		$main_network_id = 1;
	} else {
		$_networks       = get_networks(
			array(
				'fields' => 'ids',
				'number' => 1,
			)
		);
		$main_network_id = array_shift( $_networks );
	}

	/**
	 * Filters the main network ID.
	 *
	 * @since 4.3.0
	 *
	 * @param int $main_network_id The ID of the main network.
	 */
	return (int) apply_filters( 'get_main_network_id', $main_network_id );
}