lxc/lxd are ~container managers, the kernel is still shared like docker containers. It runs on linux & needs to run in a linux virtual machine on macOS.

smolvm is a virtual machine with the ergonomics of containers like packaging and distributing, kernel is not shared. You'd run containers inside of smolvm.

Also, smolvm runs ontop of both kvm for linux and apple's hypervisor for macOS. So it's cross platform.