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People also need to keep in mind that there are machines that bridge the gap between manual and CNC. For instance, there are tracer machines that would mechanically follow a 1:1 pattern of the design you want milled. So you could have your very best machinist make the pattern (at great cost of time and money, I assure you), and then the tracer could reproduce it.
Also, I’m no car expert, but isn’t it very possible that older transmissions weren’t this compact and intricate? And would therefore be easier to machine?
EDIT: I was on mobile before, and didn’t watch the video, nor did I take a super-close look at OP’s image.
Both the valve body shown in the video, and the OP are very clearly cast, and you can see the casting surface finish in the grooves. This means that the grooves were not machined at all, but were molded in their current forms. The top surface of the casting was machined so it is very flat, but the grooves haven’t been touched by a machine.
Now, the die that made it (or sandcasting form, but given the intricacy, I’d go with die) would be very impressive and intricate, but you only need to make one of those to get a couple thousand of the finished parts.