Or does he need to cast/forge/mill/whatever the frame and stamp his own name on it...
Feds define manufacture as 80% completion of the receiver.
Many people build their own handguns and rifles from cast or forged receiver blanks that don’t have all the holes drilled and tapped.
They take it over the 80% (I have no idea how the BATFE would figure that out), so they’re considered the manufacturer.
Even importers of receivers, like Springfield Armory, import the unfinished receivers (Brazil) and finish the work in the US.