To begin with, it's definitely not easy to install SteamOS in VirtualBox. Valve clearly didn't intend for you to do this; you can't just mount an ISO and install it, like a normal Linux distro.

If you want to install Ubuntu in VirtualBox on your Windows 11 computer, make sure to enable Hardware Virtualization from BIOS and then follow this guide.

Jack Wallen shows you how to install the latest major release of the VirtualBox virtual machine manager.

The VirtualBox Guest Additions gives a performance boost and extra features to your hosts. Follow these steps to install this package on a GUI-less Ubuntu server guest.