Virtualbox's Remote Display Extension (VRDE) is described in
https://www.virtualbox.org/manual/ch03. ... te-display and
https://www.virtualbox.org/manual/ch07.html#vrde
It is compatible with Microsoft's Remote Desktop protocol when the Oracle Extension Pack is installed, or a limited open-source VNC server, see
https://www.virtualbox.org/manual/ch09. ... erextpacks.
VRDE remotes into the keyboard, monitor and mouse, in a sense like a remote-enabled KVM switch, and provides remote USB and other services if the Oracle Extension Pack is used. It does not actually remote into the VM's OS, so even OS's like DOS that have no idea what remote desktop is can be remoted into.
You access the VDRE by using the host's IP address and the port number chosen in the VM's Display settings, Remote Display tab. Enable the server, pick a unique port number not used elsewhere on the host OS.
In the host RDP or VNC viewer, go to
localhost:portnumber or
127.0.0.1:portnumber.
If you want to access VRDE from other computers, open the chosen port number in the host's firewall, then on the other computer's RDP or VNC viewer go to
host.IP.add.ress:portnumber.
You can also get access to VRDE by opening a desired port on your network router's Port Forwarding rules, pointing at the host IP address and the chosen port number. Out on the internet PC go to you.public.IP.address:portnumber or your.domain.ext:portnumber.
There are authentication schemes to restrict to desired users.