This begs the question: Is it illegal to design a nuclear weapon? Or any weapon for that matter, sounds like hell of a hobby. Could you outsource a modern day Kalashnikov to the third world through the daily Slack and Zoom?
Not really, there is a big framework of regulations designed specifically to make that not a thing - from international treaties to local laws. They cover all sorts of things including the materials, the development and trade of adjacent and dual-use technology and industrial capacity, etc.
