[Thrymr]

The precision implosion of the type used on the plutonium core of Trinity (and then Nagasaki) isn't exactly trivial, but a number of nations have pulled it off. I believe the details of the precision explosives and timing for Trinity are still classified.

The Little Boy gun-type detonation of enriched Uranium is indeed fairly trivial, and end for its first use at Hiroshima they were confident enough to deploy it without full scale testing.