Please elaborate. There are literary step-by-step videos on how to build these. E.g. https://www.youtube.com/watch?v=g3LT_b6K0Mc

The covered materials are very broad, though often limited to equipment built "for purposes of", like in this section.

Title 22 Chapter I Subchapter M Part 121 - The United States Munitions List - Category XI Paragraph b

Electronic systems, equipment or software, not elsewhere enumerated in this subchapter, specially designed for intelligence purposes that collect, survey, monitor, or exploit, or analyze and produce information from, the electromagnetic spectrum (regardless of transmission medium), or for counteracting such activities.

Do share some more details please

The explanation may be spelled ITAR.