Since they own their own data centres they can pretty much control every aspect of it. My guess is they have dedicated staff that literally all they do is add more disk space. Like order/assemble/rack/plug in systems. They probably have custom setups where they snap everything together, put in like 60 of the highest TB drives available in a single 4U chassis, and slide it in, plug it in, then the cloud system takes over from there. probably have like 10+ people who do this non stop all day. When they run out of floor space they build a new data centre. At some point they probably take down older storage servers to replace with newer higher density drives, and just repeat.
But it is mind boggling just how much data they do produce so I do wonder how they do in fact keep up, it seems even if people are putting together storage pods day after day it would still not be fast enough.