It was z, not Power, but I really enjoyed STH's recent video going through the design of the z17 mainframe with IBM folks: https://www.youtube.com/watch?v=C8oLfMXUo0U

That was really interesting! All that proprietary hardware is mind blowing. The cost of those mainframes must be mind boggling, let alone the maintenance costs. It answers the question, "What does IBM do nowadays?"

But one thing I never knew about before was covered in just a few seconds in the video: 40+ layer motherboard PCBs. Holy cow, it was like a half inch thick! It makes sense now that I think about it considering how many components are slotted into it, but for some reason it never dawned on me that you could have a PCB that was so huge vertically.