If you've got an old PC sitting around, Proxmox is probably the best thing you can put on it. It's free, it's open source, and it turns that machine into a proper hypervisor running VMs and LXC containers. We actually use Proxmox almost daily here for testing. On our site we have a full beginners guide covering the install, spinning up an LXC with NetBird in it for remote access, plus ZFS pools and IOMMU for GPU passthrough once you're ready for the fun stuff.