That's an interesting point. It's easy to mount an ISO in VirtualBox, but I don't see a way to mount any sort of "virtual USB flash drive". I wonder what's the easiest[1] way to install an OS in a VM if you don't have an ISO.
Even more interesting is EasyOS's official install instructions[2] seem to tell you to download a live CD ISO for an old version, and move from there to the latest version. I guess they're not completely free of needing ISOs just yet.
[1] Obviously excluding things like virt-install. I'm talking about live booting an interactive OS or installer.
[2] https://easyos.org/install/easy-frugal-installation.html, search for "Easy live-CD"