Is the last part really true? I think large companies often hire much more broadly and some don't even have a specific position/team for you until you go through the matching process after you start the job.
My role as a contractor was always highly specific, and so were the roles of my more permanent co-workers. That being said they did tend to switch to mostly unrelated roles every few years or so.
I have a friend who worked in accounting for a large company and complained that nowadays large corporations slice responsibilities into these little bits, because it's easier to train for them this way, but the side effect was that her role had a very narrow scope and she was easily replaceable.