Virtualisation

Our Mac is stuck on Mojave because I use Softpress Freeway to maintain my website and there is no prospect of any upgrade path to a 64 bit app. In all the talk over the years about running Windows or Linux under virtualisation in a window on the Mac desktop ( e.g. with Virtual PC or Parallels), I don't recall anyone mention of running a second version of Mac OS in this way.
If this were possible it would surely be a more convenient option than having to reboot the machine every time. Is it possible, or is this just a crazy idea brought on by the heat?