That's interesting. If 'Fat Man' was a plutonium bomb, what was 'Little Boy?' Or what made it different from the Trinity device?
Trinity and Fat Man were both plutonium implosion devices. Little Boy was a gun type uranium bomb.
Little Boy was a very simple design. Take two chunks of sub-critical U235. One piece is the target, it sits at the end of a gun barrel. The other, smaller piece is shot down the barrel with explosives. When they hit, kablooey!
Plutonium implosion weapons compress a sphere of Pu239 using explosives with impeccably precise timers. The dozens of charges needed to perfectly compress the plutonium have to all go off simultaneously or the weapon will be a dud.
The added complexity of the implosion weapon made testing critical. Implosion weapons have the advantage of needing far less fissionable material so they can be made much smaller. All our current weapons are implosion weapons.