global menus made from master site’s custom menus wpmu network

That’s an old question, if someone like me landed on this page for WORDPRESS MULTISITE MENU sharing across all network sites without any plugin,

Not only menu you can use the same method to share anything other then widgets across all the network sites.

here is the solution : Edit your Header.php

//store the current blog_id - Use this function at the start of the function that you want to share

global $blog_id;
$current_blog_id = $blog_id;

//switch to the main blog which will have an id of 1
switch_to_blog(1);

//output the WordPress navigation menu - incase of menu-sharing use this

wp_nav_menu( 
   //add your arguments here
);

//switch back to the current blog being viewed - before ending of the function

switch_to_blog($current_blog_id);