Making a nuclear bomb has nothing to do with the knowledge of its' construction. Detailed plans are freely available to anyone who is interested. The reason you can't make a nuke is that the enrichment process of a suitable amount of fissile material requires nation-state level of industrial output. It is physically impossible for a small rogue actor to make a bomb from scratch. Germany during WWII, for example, was far advanced toward a bomb years before the Manhattan project, but their industrial capacity was simply never sufficient to build it.
I would STRONGLY disagree that the Germans were ever more advanced than the Manhattan project. The British were a bit (~9 months) ahead of the US before S-1 started (December 1941), but I've never seen any evidence that the Germans were ahead of either country at any point. Certainly the fact that they never managed to replicate CP-1, which went critical in December 1942 (and was still years of hard work away from an actual nuclear bomb) suggests that they were never very close at all.
The closest I can come up with is that the discovery of nuclear fission in 1938. In that work, there were five people who collaborated closely enough that today they would have all had their names on the one paper (because of Nazis the work was published in two papers, Aryans on one, Jews on the other): Otto Hahn, Fritz Strassman, Lise Meitner, Otto Frisch, and Wilhelm Traube. Hahn and Strassman were considered Aryan enough to stay in Germany. Meitner and Frisch escaped- Frisch ended up working on the MAUD report and then at Los Alamos, Meitner stayed in neutral Sweden. Traube did not manage to escape and died in Gestapo custody in 1942. So 20% of this team ended up at Los Alamos, and only 40% managed to stay in Germany, which does somewhat point to some of the underlying problems any German bomb project would have had.
Yes, but you don't know if those plans are viable and correct. This whole thing about the gun firing a cylinder is a case in point. People *thought" they knew how it was built sans some detailed measurements and such, but it turns out they were completely wrong on a rather critical aspect of the design.
Is this still true in the 2020s?
You still need thousands of kilograms of natural metallic uranium, which is a rare heavy metal with few commercial uses and one really salient illegal one, or thousands of tons of uranium ore, which, see previous. It's hard to buy this stuff secretly, because not many people have it, and people who do have it have a big, salient, vested interests in not selling it to people who want to make nukes! It's not like selling drones to Saudi Arabia and saying "oh well guess it'll suck to be a Houthi". You might get nuked!
Nation-states can pull this off, since they have both land and state security apparatuses. Empirically, private groups or terrorist cells haven't been able to do it. The rumors I've read and the impression I've received is that AQA spent decades trying, lost a lot of lieutenants and trucks full of cash to CIA honeypots, and eventually gave up. Too hard, too expensive, too many dead ends and entrapment schemes.
To enrich uranium legally in the US you would have to get a license by justifying why you were doing it and there are not many legitimate reasons to do so outside slightly enriched uranium for power plants or highly enriched for a few medical isotopes.
But if you just decide to do it, yes it’s illegal and the NRC will probably find you.
The laws of physics haven't changed in 80 years. Even if you started with yellow cake uranium (~70% enriched, and itself already nearly impossible for a non-state actor to acquire), to reach weapons grade at >95% you'd need hundreds of tons of it, and massive industrial scale chemical facilities to convert that into uranium hexafluoride and pull out the U235 isotopes [0], where the ratio of U235 to U238 (the non-fissile isotope) is 99:1. The vast majority of the Manhattan Project was in the engineering challenges required to do this, not really in the construction of the bomb. So far, only 5 countries in the world have been able to do it.
[0] https://web.evs.anl.gov/uranium/guide/uf6/index.cfm#:~:text=....
I think you’re a tad optimistic there - sadly we have many more than 5 nuclear powers today.
Another possibity is building a breeder reactor first to breed Plutonium.
Also, where to get Uranium ores in the first place? You can be sure that governments keep close tabs on who takes funny stones out of certain mines.
