In the late 1990s, when I joined a bank that used IBM mainframes, there was a group of 30ish new CS graduates who would spend many months being trained internally and by IBM to learn to program and operate those. As I understand it, that's still the case today.

The people who operate these mainframes have tremendous access to internal IBM resources. I remember having to stifle my laughter as, during a meeting about a major outage, a seasoned programmer explained that he "took a dump and sent it to IBM" and was expecting to hear back any time.

The channels that exist for this knowledge are expansive and just aren't on the internet, though they do occasionally spill over there.