Virtualbox guest additions allow better integration between the host and guest operating systems like seamless copy/paste, flexible screen resolution of the guest os etc.
After installing an OS inside VirtualBox you should install the Guest Additions right away to get the best features and functionality out of it.
It installs driver programs that enables native resolutions same as you current host OS, and also enables copy paste across the host and guest OS.
After installing the guest additions you can check if the guest additions are properly installed and active or not.
If your guest OS is a linux like Ubuntu or Fedora and you want to check if virtualbox guest additions are installed or not then you can use this simple command
$ lsmod | grep -i vbox
If the vbox modules are loaded they're installed and working. If nothing appears then probably virtualbox guest additions are not installed.
The output could be something like this for example
$ lsmod | grep -i vbox vboxvideo 49152 0 drm_ttm_helper 16384 2 vmwgfx,vboxvideo ttm 94208 3 vmwgfx,vboxvideo,drm_ttm_helper vboxguest 385024 5 $
or something like this on older systems:
$ lsmod | grep -i vbox vboxvideo 12612 1 drm 264190 2 vboxvideo vboxsf 43659 0 vboxguest 214867 7 vboxsf
If the output of the lsmod command does not show vbox modules, then either guest additions were not installed properly or they are not loaded. Then you might need to debug further.
To check the module details we can use the modinfo command as follows:
$ modinfo vboxguest filename: /lib/modules/6.0.17-200.fc36.x86_64/misc/vboxguest.ko version: 6.1.38 r153438 license: GPL description: Oracle VM VirtualBox Guest Additions for Linux Module author: Oracle Corporation srcversion: FA5DE49C7F2CB39107D8436 alias: pci:v000080EEd0000CAFEsv00000000sd00000000bc*sc*i* depends: retpoline: Y name: vboxguest vermagic: 6.0.17-200.fc36.x86_64 SMP preempt mod_unload $
The above output shows the exact module driver file name which in this case is
/lib/modules/6.0.17-200.fc36.x86_64/misc/vboxguest.ko and other details like the version and vendor details.