VirtualBox (along with probably all/most virtual machine software) stores the guest OS's files in a file (or series of files) instead of literally saving the files on the host.
I'm unaware of if you can use any sort of tools to open these, and it definitely would depend on the type of file, but from my searches in the past there aren't any for VirtualBox.
Luckily, VirtualBox has shared folders. If your guest OS supports the VirtualBox Guest Additions, install them and then set up the shared folders tool thing. Put files you want on the host in there, and you can access them on both.
Alternatively, you can use the shared clipboard feature.
If your guest OS doesn't support guest additions, you may be out of luck. The only solution I can think in that case of is some sort of external storage (server, flash drive, etc.) that both can use independently of the virtual machine.
