My understanding is that it isn’t enough to know how to build a nuclear weapon, or even to have the materials. Uranium and plutonium will kill you very quickly in a machine shop setting if you don’t also have the right protective equipment and a whole lot of practical experience.
Don’t underestimate how hard it would be to assemble an actual working bomb.
It depends on your goal. If you want to build the bomb and survive in the long run, it will be difficult. If you just want to make a big bang with no other requirements, unfortunately it's pretty easy to do.
We could be happy about the two things that it is not that easy to get enough plutonium in the required purity and that most people love their lives.
Is it possible that most terrorist groups understand the cost/benefit trade off? In other words, if you have folks capable of designing and/or assembling a nuclear device, those aren’t people you just throw away.
I understand that terrorists don’t value lives highly, but you don’t see experienced bombmakers actually conducting suicide attacks for a reason. They’re valuable alive.
You need to run your machine shop like a clean room. Laminar airflow, high grade air filters, glove boxes, etc. Anyone who has ever even walked through a machine shop will know how challenging that would be. You probably aren’t going to do it in your garage.
For uranium there’s an actual public US government report.
Don’t underestimate how hard it would be to assemble an actual working bomb.