bbowens wrote:Mac Mini
If this is an M1 Mac, then Virtualbox does not work on M1 Macs. If it's an Intel Mac, you might be able to proceed.
As for disk imaging to bring the Mac OS into a VM, you're trying a physical-to-virtual, or P2V. Web-search "P2V Mac Mojave" or something similar to find out how it could be done.
One idea that comes to mind is to run backup software in the Mojave Mac that can image the whole drive to a backup image on an external USB drive. Then copy that image from the external drive into a mounted virtual drive file on the Monterey host. See
https://www.virtualbox.org/manual/ch05.html#vdidetails for the kinds of disk images Virtualbox can use. If Monterey is capable of mounting one of these formats so it looks like another drive in the computer, then you can copy the image file to the mounted virtual drive. Then unmount it from Monterey, and attach it to a new Mac VM as a secondary drive. Boot the VM with the backup software's restore media, and restore the image to the Mac VM's primary drive. Then cross your fingers and try the VM.