WRT metal cranks, there's really no difference in production between a metal crank and a comparable tool.
They can be cast or forged. Some have a hollow core to reduce weight.
carbon fiber ones are trickier. Layers are laid up in molds, incl threaded inserts for pedals. Resin is injected(unless the cloth is pre-preg) then the whole thing is cured in an oven or autoclave.
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