The movie sort of treated the bomb like oh it really worked.
That was the plutonium implosion bomb. Implosion was required for the plutonium bomb because plutonium had too high a spontaneous fission rate to use the gun mechanism. A lot of theoretical work and engineering went into understanding implosion before a working bomb could be built.
The enriched uranium bomb, however, was considered to be such a sure thing that it wasn't even tested before it was used on Hiroshima.
If you look at the overall history of the Manhattan Project one thing stands out--the numerous unknowns forced the project leadership to hedge their bets by using multiple different techniques simultaneously. For example, plants to separate uranium using gaseous diffusion, thermal diffusion, and electromagnetic separation were all built on industrial scales. And an entirely separate infrastructure was built to produce plutonium and chemically separate it from the reaction byproducts.