You get a virtual machine, so you can build and run untrusted software. If you have very specific old software (for example, something that runs on Ubuntu 12.04 LTS), then you can install LXD inside a Multipass VM, and from LXD create a system container with Ubuntu 12.04.

While Multipass supports LTS versions of Ubuntu and recent development versions of Ubuntu, with LXD you get container images for many more versions and other distributions. See the list at https://us.images.linuxcontainers.org/

Make sure your hypervisor is up to date. It's uncommon but vulnerabilities that allow applications in the guest VM to escape the hypervisor and infect the host do happen sometimes. For the most part as long as your hypervisor is up-to-date this would be a great solution for you to use untrusted software. Just keep in mind the "primary" machine gets special connections to the host, which can include file sharing and hardware sharing, etc. Do it in a machine without those things for increased safety.