The big issue for most countries is getting the fissile material in a form that’s useful in a nuclear weapon. There are two types of fissile material useful in bombs- plutonium 239 and uranium 235.
U235 is a very small constituent of natural uranium, so huge quantities of uranium have to be isotopically separated via centrifuge, calutron or gaseous diffusion, leaving large amounts of U238 behind (“depleted” uranium). As you can imagine, this is a huge, industrial scale process that’s not easily hidden, nor is it cheap.
Plutonium is generated in working nuclear reactors from U238 through normal reactions. Pu239, the useful isotope is created, along with Pu240, which is a contaminant that emits a lot of spontaneous neutrons and causes predetonations. So if you’re trying to produce plutonium, you want short fuel cycles to minimize Pu240 generation. This is expensive as well.
However, separating Pu from U is relatively easy. So that’s the route most countries with nuclear reactors follow.
Once you have your fissile material, you have your design choice. U235 allows you to produce a relatively simple gun-type bomb (think Little Boy/Hiroshima) but requires a relatively huge amount of expensively separated U235 for a relatively small yield- Little Boy had 140 lb of U235 for a 15 kiloton yield. U235 is also suitable for implosion type weapons as well.
Plutonium’s inevitable contamination with Pu240 rules out gun-type weapons, as they aren’t physically fast enough to overcome the excess neutrons from the Pu240, and result in predetonations. So if you use plutonium, you’re required to make implosion type bombs, which are MUCH more technically demanding, but much more efficient. Fat Man (Nagasaki) for example, produced a 21kt yield from 14 lbs of plutonium, which was drastically more efficient than Little Boy.
Most of the Manhattan Project was spent trying to figure out implosion bombs and how to produce enough the proper fissile material(U235 and Pu-239) correctly. Most of the design of Little Boy was determined by mid-1944 (Thin Man) as part of an abortive gun-type plutonium bomb, and was actually simplified for use with U235. Manhattan Project scientists were so certain of Little Boy’s functioning that there was no test- the design was used for the first and only time in combat over Hiroshima.
The implosion method, on the other hand, was a MUCH more involved engineering task, and required a test (“Trinity”) to see if it would indeed work.
From what I gather, some aspects are much easier today- computer assistance in engineering and manufacturing makes parts more precise, for example. But certain aspects are difficult, and getting certain parts/items is actively discouraged/impeded by other countries- things like krytron/spryrton tubes, maraging steel (used in high speed isotopic enrichment centrifuges), etc…
So some countries, primarily Western industrialized ones like say… Germany, Japan, Netherlands, Italy, etc… could probably gin up a working nuclear weapon in a relatively short time- a year or two, IMO- they have developed nuclear power infrastructures, as well as highly advanced manufacturing bases. They can make their own maraging steel and krytron tubes, for example. Plus, most of the physics is relatively well understood as are the industrial processes, so they’re not having to reinvent nearly so much of the Manhattan Project’s wheel.
Other countries, like say… Slovenia or Mexico would have to develop a lot of scientific and industrial infrastructure just to get to the point where they could start development work.