I use the flatpak version of Boxes on Pop!_OS 21.04 – it works really well. I don’t like to use Virt Manager as its UI is confusing.
Is it possible to SSH into a VM with the flatpak version? Or can I connect to a port that I expose from the VM using my browser? It seems to be doing NAT over the host. It can reach the host but not the other way around.