I know how suspension bridges work. I probably could build a small one, but any lengthy span would be well beyond me.
I know how internal combustion engines work. It would take a year of training on the tools before I'd be able to make one that even sorta worked, and then it would be at 1900s-level functionality.
I know how nuclear weapons work. Several types, in fact. But I cannot make them.
1) I could build a gun-type weapon, given the material (200lbs of 90% pure U-235, a 76mm artillery barrel, and some regular explosives), but I could not create the equipment to refine uranium.
2) I could probably build a reactor to generate plutonium, with massive effort and a significant risk of poisoning myself, but I could not build a working implosion bomb with it. It would take a year's training in explosives just to be able to build an existing design, and those designs are tightly secured.
3) With the materials, I might be able to upgrade an unboosted fission weapon into a boosted one. Maybe.
4) A fusion weapon is completely beyond me. You could stick me in Lawrence Livermore with all the parts in front of me, and without some Ikea-like instructions you aren't going to get anything.
We are protected from homemade gun-type weapons by the scarcity of uranium and the immense difficulty in refining it. Remember, this is something that was beyond the capabilities of most nations a scant 70 years ago. A dedicated nation-state or perhaps certain multinational corporations could pull it off, but not without detection.
We are protected against homemade implosion-type weapons by the complex engineering necessary, the esoteric nature of the specific engineering knowledge needed (nuclear physics and shaped explosives are not a common dual-major), and by the absolute need for testing before use. The former prevents fringe groups from succeeding; the latter prevents the non-suicidal from trying.
We are not protected by lack of general knowledge on nukes, because no such lack of knowledge exists. I learned half of this stuff from school textbooks, and the other half from Wikipedia. Anyone driven to find more can easily do so.