*** host mac, virtual machine guest Ubuntu, share folders

See
https://youtu.be/TcrfrVNNGMU

First, need to run "virtual box guest additions  CD image", because default Ubuntu distribution does have the guest access feature.

After add shared folder in VirtualBox, check "Auto-mount" and "Make Permanent"

in Virtual Ubuntu, $ sudo adduser hqin vboxsf

The restart virtual Ubuntu. Shared folder appear as added drives.