Fairly sure it requires you to have your own copy of Windows; it doesn’t provide one for you. Otherwise it would be a massive licensing violation / piracy. If you look at similar projects like WinApps, that’s how they work.

Seems to be an evaluation license... that said, you can run recent versions of windows unregistered for pretty much as long as you want... they disable some UI enhancements, like changing the desktop, but it tends to function.

Not sure on older versions... MS used to generate new evaluation ISOs every few months with a hard fuse in them... Can't speak for recently since it's been years since I've needed to test for old IE versions. If you choose to customise, I'd suggest going with one of the "lite" modified versions of Windows 10... since this will reduce the default surface a lot without the bridge to some of what Win11 added while jumping the shark.