It was released (a major one) with paravirtualization support for Windows and Linux guest machines, more hardware-assisted instruction sets for virtualization and nested paging, drag’n’drop support out of the box for Windows, Linux machines and more.
Note that guest additions are installed on guest OS (hence “Guest” additions).
Therefore, the guest additions updates are also installed on the guest OS and not the host OS.
This can be automated by a script, but by default, if you reboot your host, your guest machines will not be running any more.
In the bottom right hand corner of the emulated machine window is the name of the key required to make Virtual Box un-grab the mouse and keyboard.
By default this is Control_R which actually means the right hand Control key, not control and the letter R.