Squad--The basic infantry unit?
This may be utterly obsolete information--I'm going 'WAY back to 1950s Army ROTC stuff. There are some military historians on TFL who could--and I hope, WILL--give beter answers. USMC ranks, etc, vary somewhat, I believe.
In an infantry outfit--
Squad--about ten soldiers, plus or minus a couple. Led by a buck sergeant. Squad was split into two fire teans, usually four men, led by a corporal--Browning Automatic Rifle as base of fire. All rest armed with M1 rifles back then.
Platoon: Usually three to five squads. Platoon leader usually a Second Lieutenant (M1 carbine plus a .45, generally,) with a platoon SGT--What? A Staff SGT, I believe.
Company--Three or four platoons. Captain as C.O., 1/LT as executive officer, plus a platoon SGT. I believe a Tech Sgt back then. Now, I dunno.
Officers all had sidearms and carbines.
Each company had a weapons platoon, with a couple of air cooled belt fed Brownings and, I think, two 60 mm and later, 81-mm mortars. Gunners carried pistols; support personnel, ammo carriers, etc.,, carried rifles or carbines.
Vast over simplification above, of course. I'd be interested to read about current organization. Different weapons, certainly. Wonder about other difrferences.
But the squad as a unit goes way back to at least Roman times: The "decade" was ten men, led by a decurion, the ancestral sergeant.
Best regards,
Johnny