Yeah, there’s an entire ecosystem of machine-builders and process-designers and parts-suppliers for any mass-production setup. The product I write manuals for… we designed the plastic shells, and then went back and forth with a supplier to design the moulds. Another supplier provided batteries, and they had their production setup. We bought off-the-shelf displays, and the display maker has their setup. A company just down the road did the circuit boards, and they had all sorts of machines to do that. And the circuit-board maker called on people who make the chips and electronic components themselves. Other companies would make metal parts and brackets to order.
In this case the final assembly was by hand, but even then there are all sorts of jigs and test fixtures that we would make in house to ensure that the workers could do the same thing repeatable. (I used to program some of the electronic test equipment.)
Really, when you think about it, the complexity of interrelated suppliers for any moderately-complex item like a cellphone, say, is staggering. And then consider the making of cars…