99.9% uptime? / upgrade?...

Hello,

I wondered how do big companies upgrade they database system without down time?
I understand high availability stuff like RAC,Data guard etc ...
But can’t get my head around of transferring data from system a to new system (b) or upgrading system a with no down time, it’s possible at all? I have seen people claiming 99.9% up time ...is this is the other 0.01%?
:D

Our SLA is also 99.9% up time, here is how it works... 99.9% up time PLUS scheduled maintenance windows time.

You do your stuff during maintenance window.