Fat Man had a “soccer ball” type trigger- HE in a ball shape imploding the core to compress and start the reaction.
Little Boy was a “cannon” type- a tube with the nuke material being driven like a piston to initiate.
Maybe OM can tell us the answer to Your question.
Fat Man was an “Implosion-type” weapon using a hollow sphere of Plutonium that was surrounded by shaped charges of high explosives. The shaped charges were designed to created a near-perfect implosion wave that crushes the Plutonium sphere to reach critical mass.
Little Boy was a “Gun-Assembled” type weapon that used enriched Uranium. A slug of this Uranium was placed at the end of a cut off gun barrel and fired down the barrel. The other end of the barrel had rings of the Uranium as the target and when the slug entered inside the rings it created the necessary critical mass. A very simple design.
Neither bomb dropped on Japan was an H-Bomb (actually called a Thermonuclear Weapon) as the very first Thermonuclear detonation was on 1 Nov 1952 named Operation Ivy-Mike. The Ivy-Mike design was the brain child of Dr. Edward Teller (a great man that I have had the honor to meet several times while I worked at the Labs).