Creating groups with debian is weird, because it does not have the usermod utility mapped. you must execute /sbin/usermod instead.

As opposed to Ubuntu 18.04, Firefox is ESR instead of latest.

https://linuxhint.com/install_kvm_debian_10/
The above link is pretty much exactly how you would want to go about installing qemu. The exact instructions are written below.

Install qemu.

$ sudo apt install qemu qemu-kvm qemu-system qemu-utils

Install libvirt to manage VMs.

sudo apt install libvirt-clients libvirt-daemon-system virtinst

Check to see if the libvirt daemon is running, if it is not, start it.

$ sudo systemctl status libvirtd

Enable the default network, check if it is on and enabled to autostart.

$ sudo virsh net-list --all
$ sudo virsh net-start default
$ sudo virsh net-autostart default

Install libvirt-manager

$ sudo apt install libvirt-manager