They’re OSes sort of in the way a Linux Distro is.

Yes, not in a container. Unraid won’t work well in a VM (Proxmox), but TrueNAS will. Both are really intended to run on raw hardware though.

They’re both primarily systems for doing NAS - managing multi-drive storage, sharing with other hardware on your network. But also make it easy for you to deploy containers and VMs on the same hardware.