We got lucky that the nazis banned and expelled legitimate science and scientists on political grounds. This ensured that their science progress stagnated thus preventing them from acquiring nukes in time to change the course of the war.
Not trivial to make without the resources of a major industrial nation, but the actual simplest mechanism is essentially just sufficiently purifying two lumps of a metal of sufficient size and quickly smashing them together.
You don’t get modern technology, say the transistor, without nuclear inevitability.