How to access files created from VirtualBox in thunar (or any file explorer) in Debian Linux?

The easiest way would be to start the VM in VirtualBox and either open Google Keep through Windows, or use e.g. VirtualBox’s “Shared folders” to copy the files out. While the VM is running, you also should be able to use the “File Manager” tab in VirtualBox’s main window.

If you cannot boot the VM, find the disk image in Virtual_Machines/ComputerOne/ – look for the a large *.vdi or *.vhdx file that represents the VM’s entire system disk. (As long as you haven’t deleted the VM, its disk image will be somewhere; try find /media/earthly -name '*.vdi' to find it, as well as find ~ -name '*.vdi' in case it happens to not be on the external disk.)

Once the image is found, install the qemu-utils package, then run:

sudo modprobe nbd
sudo qemu-nbd -c /dev/nbd0 Virtual_Machines/ComputerOne/ComputerOne.vdi

Check with lsblk and lsblk -f that nbd0 now shows up as a non-empty disk with partitions. You should be able to access the VM’s disk using mount or through Thunar.

Afterwards, disconnect the nbd device and (optionally) unload nbd entirely:

sudo qemu-nbd -d /dev/nbd0
sudo modprobe -r nbd