I think '100% uptime' is pretty much shorthand for "absolutely no reason to turn the machine off or stop services on them running, ever".

As the above link mentions, everything is redundant. CPUs running in (near) lockstep - voting bad cpus out, redundant storage, power etc etc etc.

I've not seen a better write up on IBM's equivalent systems - but I've not had a reason to play with such big iron. NSKs however were much more affordable, and were used in the late 90s/early 2000s in various telcos etc.