Virtual Machines

Even if you have a macOS installer package, and genuine Apple hardware, installing macOS in VirtualBox fails in ways that are nearly impossible to troubleshoot. Fortunately, someone made a shell script which handles all the weird hacks necessary to make it work.

It takes a long time to download and install (and can't use any of the installer packages you might already have sitting around). It takes a ton of disk space to install (>50GB), though there's a way to run the script later to reclaim some of this. The graphics are always too small or too big. My experience is that it runs CPU-bound tasks at about 50% actual speed, and GPU-bound tasks much slower. It's good enough for "One user reports a bug under macOS version X that I can't reproduce under macOS version Y".

In the long run, it's better to use actual installs, on separate partitions (APFS might make this easier?), or external disks. Beg/borrow/steal a ≥50GB disk (spinning rust is fine), partitioned in 2. Between that and your main system, you can have complete installs of 3 major versions.