This question already has an answer here:
What happens if I install a virtual machine additions package (e.g. Virtual Machine Additions for Microsoft Virtual PC 2007, Integration Components on Windows Virtual PC, and Guest Additions on Oracle VM VirtualBox) on a host PC? Will there be any adverse effects, or will my system work fine?
In other words, what happens in the backend on the virtual machine when I enable integration features like mouse pointer integration, window resizing, and folder sharing?
Disclaimer: I know that it is possible that this question could be marked as a duplicate of this one, but that one is asking about Ubuntu and VirtualBox specifically. My question is more general, and it also asks about how integration is internally implemented.