If they are all developing with Xcode I bet they also have a server farm for distributed builds. That said, why would OSX devs need more that 16GB of RAM? It's not like they are going to be running virtual machines/docker images. Xcode isn't _that_ much of a memory hog.
What are they building Xcode server farms out of?
Apple Stores /s
Why wouldn't they be running VMs? I frequently use them for development, within OSX.
The 27" iMacs are all configurable up to 32GB RAM.