There is a veil of secrecy to the devices (identified here: https://meridian.allenpress.com/radiation-research/article-a...) which makes them difficult to study, however millimeter waves themselves are studied: https://ieeexplore.ieee.org/abstract/document/10590163

While looking around I also found this handbook: https://books.google.se/books?hl=en&lr=&id=OVRFDwAAQBAJ&oi=f... which is quite informative.