One of the things that blew my mind in my early career was seeing my mentor open the side of a VMS machine (I can’t remember the hardware model sorry) and slide out a giant board of RAM, and then slide in another board the same physical size but it had a CPU on it, and then enable the CPU

The daughter board in that machine could have RAM or CPUs in the same slot and it was changeable without reboots!

Exactly! One would never, ever do that with x86.