AMT is is hardly a secret. It is a feature for IT departments to get pseudo-console access to PCs over the network and remotely manage them. (The common term is Out of Band or OOB management.)
It is comparable to HP’s iLO or Dell’s iDRAC, which are for servers. In the case of servers, there is usually a dedicated NIC for OOB. For desktops, that is not feasible. So Intel’s AMT lets the motherboard NIC be shared between the CPU and the Management Engine.
Here are some links to get you started:
https://software.intel.com/en-us/articles/getting-started-with-intel-active-management-technology-amt
https://www.howtogeek.com/56538/how-to-remotely-control-your-pc-even-when-it-crashes/
Note that despite many of the articles you read, not all Intel PCs have this feature. It is typically only found on business CPU/chipsets, as it is one of the differentiating features to justify the price point.