It's (largely) a change of thinking: "When I want to automate the production of this object, what reduction in number of components / assembly complexity can be done".
The one-piece milling is a typical example. Using CNC, you can create a complete wing-spar, with whatever additional items already attached, etc. Creating such a piece in manufacturing is a matter of "let the machine run", and no longer individual craftsmanship on a lot of individual components, requiring another site of skilled craftsmanship to assemble.
Think about a 3D printer, which easily creates 1000+ versions of the same, once the model is created. Compare that with a lot of individual injection molding parts, which need to be assembled later on, each with its tolerances, etc, to get the final object.
Noted, that 3D printing does still have its challenges regarding long-term durability, etc.
Last edited by WideScreen; 17th Apr 2023 at 11:06.