It’s hidden inside the xhyve virtual machine. But you don’t really need to look inside it. If you’re really curious you can use the magic screen command to get a shell in the VM, but mostly it’s all internal Docker details.
If you’re trying to access the volume data, I think the usual way is to launch another container: docker run -v test:/test -it ubuntu:16.04 bash will get a shell with the volume data visible in /test.