There are lots of information about Type-B messages sent by airlines on IATA website but they have not stated under what communication systems (cable, teletype, intranet, internet, etc.) these messages are exchanged. What are these systems?
It's not just one. Private networks are used such as the IP-based ARINC AviNet, AFTN Teletype, etc.
IATA's Passenger Data Exchange Support Plus package provides governments with all necessary standards, helpfully grouped in a single purchase.
The following example is for France (may not be current):
There is also a presentation on the IATA website you may want to download and check.