I’m just remembering this off the top of my head, so it might be somewhat inaccurate... Here goes:
One of the trickiest things that they had to develop to make the implosion device was a way to reliably trigger the spherical explosives at the right time and in the right sequence, so that the needed spherical implosion wave had the proper configuration. In the early bombs they had 32 points of detonation to trigger, making this a non-trivial task. They needed to coordinate the triggers within 10 nanoseconds (I think).
Anyway, this was way beyond what was commercially available at the time, so they developed their own triggering mechanism. They did so in the time needed (I think the triggers were only available mere weeks prior to the first test). The method used was an exploding bridgewire detonator, and was one of the most top secret parts of the whole endeavor. The work was done by Luis Alvarez and Lawrence Johnston.
I got most of this from the book by Richard Rhodes. Also, here is a link to a wikipedia article: https://en.wikipedia.org/wiki/Exploding-bridgewire_detonator
The entire story is absolutely astounding isn’t it?
The story also goes that development of the B-29 was an undertaking of similar magnitude though I find that hard to believe.
Hanford works alone had 50,000 laborers at one time and most had no idea what they were building.
I knew a guy from a town in Western Oklahoma that was nabbed right after he graduated as an electrical engineer from OAMC. They put him on the group responsible for provision of electrical power grids at Los Alamos and told them if the power ever went out they would all be dead. Probably a little drama there.