Serious question: How a military networking infra works?

I've been a (somewhat) Sysadmin in the Israel army but I've never been able to understand how the infra actually works under the hood.

How are they able to 'encrypt' and manage all the data the goes inside the network?

How is it possible for them to use it as a private network that does not go outside, and i know they do have some solutions to get things from inside the network to the regular internet network (such as documents and files) and vice versa.

It'd be very nice to understand how is it working. I've never been a networking guy myself but this subject sounds cool and unique to me.