proxmox is using KVM for virtualization and Linux Containers (LXC) for the containers. I agree that something like terraform and/or ansible would make more sense for an IAC (infrastructure as code) deployment. Most of the people I talk to that use proxmox for a homelab prefer to do things manually and don't bother with any IAC implementation.

For work I'm a firm believer in reproducible environments and IAC. We actually a combination of vagrant, libvirt, and KVM to spin up local clusters for quick testing and development. It works out pretty well, but in my homelab I don't have anything complicated enough to bother setting up terraform/ansible for. Although I imagine if my server crashed I probably wouldn't think that way anymore.