Hi,
for each VM, you can configure the VM State Storage in the VM's Options in the UI. That storage will be used for snapshots with RAM and hibernation. EDIT: The default is one of the VM disk's storages, with path-based local storages being preferred.
When hibernating, nothing extra is written to the disk files of the VM. This is not necessary since the full state is saved during hibernation including pending IO and all RAM.