It’s a function of ZFS itself. Data that is to be written to the drives is first written to RAM, then transferred to the drives. One of the benefits of this is that if you are moving a file that is smaller than the available RAM, your transfer won’t appear to be limited to the write speed of the drives.

ZFS. It can use up as much RAM as you care to give it for caching. So if you are slinging a lot of data back and forth, more RAM is better. Especially if you are using HDDs instead of SSDs.

That would probably depend on case, motherboard, and which (if any) pcie slots are occupied.