Simple gun-type fission weapons, don't require very sophisticated physics. I heard a story about from physics professor who said: If my physics students could not do calculations for a simple nuclear weapon, I would require them to return their diploma, because they didn't learn enough physics.
https://en.wikipedia.org/wiki/Gun-type_fission_weapon
"Little Boy" was exploded in Japan without previous full scale testing, so confident were the physicists in 1945.
"Unlike the implosion design developed for the Trinity test and the Fat Man bomb design that was used against Nagasaki, which required sophisticated coordination of shaped explosive charges, the simpler but inefficient gun-type design was considered almost certain to work, and was never tested prior to its use at Hiroshima."
https://en.wikipedia.org/wiki/Little_Boy
The Nth Country Experiment:
"The experiment consisted in paying three young physicists who had just received their PhDs, though they had no prior weapons experience, to develop a working nuclear weapon design, using only unclassified information, and with basic computational and technical support."
https://en.wikipedia.org/wiki/Nth_Country_Experiment
Now in 2026, the access to nuclear weapons is restricted by restricting access to materials necessary to build nuclear weapons: highly enriched uranium or plutonium.
https://en.wikipedia.org/wiki/Special_nuclear_material
The details of uranium enrichment technology are restricted and very closely monitored.
https://en.wikipedia.org/wiki/Zippe-type_centrifuge
"The production, import, and export of maraging steels by certain entities, such as the United States, is closely monitored by international authorities because it is particularly suited for use in gas centrifuges for uranium enrichment."