I have a WP Multisite / Multi Network installation. What I need is to redirect **all non-logged** and also **all logged in** users, except logged in network administrators, from the main site _example.com_ (including all nested pages, posts, categories etc.) to the _subsite.example.com_, that is a segregated multisite network. They share the same database. The network administrator will have access to the main site only after they will log in on the _subsite.example.com_. In this way, only the logged in network administrators must have access to the main site _example.com_.

__UPDATE__

The @seamus-leahy solution works, but only partially. I use his function in a multisite multi-networks installation and all networks in this configuration works like separated WP installations, so the network _example.com_ will not recognize and log in automatically the network administrator when he/she is already logged in on the subsite.example.com (network 2), but logged out from the _example.com_ (network 1). This means that the network administrator, in this context, also cannot access the main network example.com. How to solve this?