Virtualbox - Can't install guest additions (Big Sur)
You can not install guest additions when using Big Sur. I have seen some hacks posted that are suppose to work, but I have never found one that works with a Big Sur guest.
Here are some ideas of how to substitute the functionality that guest additions may have offered.

Screen Size: The command given below can be used to change the video resolution of the Big Sur guest. For more information, see section 3.14.1. Video Modes in EFI of the online Oracle® VM VirtualBox® User Manual. You can also download a PDF version from here.
 VBoxManage setextradata &quot;VM name&quot; VBoxInternal2/EfiGraphicsResolution HxV

You should enter this command with the guest is powered off.

Transfer Files by Network: You can use SMB to connect to the guest by the host or any other computer on the LAN. You should make sure the Attached to: for the network is set to Bridged Adaptor.

Transfer Files by USB: You can attach a physical USB flash drive to the guest. To do so, you must first eject the flash drive on the host. When the guest powers off or ejects the flash drive, the host will automatically attempt to mount any volumes on the flash drive. If the host has USB 3 ports, then USB 3.0 (xHCI) Controller should be selected under setting for ports.



Note: VirtualBox defaults to SATA HDD for the guest. You can change this to to SATA SSD or NVMe SDD. Only NVMe SSD has trim capabilities, which I have found to increase boot times. (Booting is when trim operations are performed.)

Perhaps, a better solution would be to avoid VirtualBox all together. VMware now offers the use of Fusion Player for free. You do need to create an account to get a license key. VMware Fusion Player is both Big Sur host and guest compatible with Intel based Macs. Also, installing is simplified. You only need to drag and drop the Install macOS Big Sur application into a Fusion Player window and the installer drive is automatically created. In other words, you do not need an ISO or flash drive to install.

Note: You can have both VirtualBox and Fusion Player installed at the same time. Although, I am not sure if using both to run virtual machines at the same time is wise.

IMO, the fact that I can not install Big Sur on my 2013 iMac, but can install Big Sur in a virtual machine on the same Mac, just seems wrong (or unfair).