typically milled on a CNC milling machine. Production done on 3 or 4 axis with built in tool changer to save time.
AR lowers are either milled from a forging or an investment casting.
A 100% milled receiver would be milled from a solid plate or billet,in which the mill cuts the entire shape out.
The original Rocky Mountain Arms "Patriot" 223 pistol was available with a milled billet upper and lower receiver. Very precise.
I think Brownells offers a milled Billet AR lower currently...in the $ 300 range due to the machine time in producing such a part.
3X the machinetime of milling out a forging which has the basic exterior formed.
There were milled AK's as opposed to folded, welded stampings.