Earlier quoted context omitted.
While John Deere does buy OEM parts, it designs and builds a lot of stuff in house, including engines. It owns and operates foundries and it even designs its own embedded computers (and writes the firmware, operating system, CAN drivers, and application software). It probably takes longer to prepare the factory for a new model-year build of an existing tractor than it takes some software vendors to get an MVP to mark…
so? they probably sell parts to competitors and vice versa. for example, a Chrysler pt Cruiser have a engine designed by bmw, made in Brazil and Mexico on foundries owned by Volkswagen. that's an extreme example, but hope this gives the idea to the downvote army.
Why are you confused? You said "all manufacturers just assemble readily available parts". I demonstrated that this is not the case for John Deere (or probably any other automotive manufacturer), which builds many of its own components and writes much of its own software in-house.