Before going further it is better to understand the following terms to simplify explanation.
These directories do not exist on disk. Instead these are created and deleted on demand in memory. If the system reboots all these directories vanish. In the previous figure, all directories under /home are virtual directories.
This is the directory that holds all Virtual directories. This directory does exist on disk and therefore it remains even after reboot. In the previous figure /home is virtual base directory.
These are the directories that actually reside on the disk. Even after reboot, these remain intact. In the previous figure all directories created under /autohome are real directories.
This is the directory that holds all real directories. In the above figure /autohome is real base directory.
Each virtual directory is mapped to a real directory. This means that whatever is written to or modified in the virtual directory is actually sent to the real directory.
On reboot of the system real directories and their content remain intact. But virtual directories are again created on demand, exactly as they were before.
Virtual directories are removed if these are not used for a specified period of time, and created again if necessary. When a Virtual directory is removed, the backup program is started on the corresponding real directory - if backup is configured.
Applications should access only virtual directories. Real directories are hidden from applications. Only the root user can see them. There is one exception: backup programs always access the real directories only.