Get it & forget you ever saw the name FlashBoot.
The FlashBoot ad copy says:
Imagine being able to install an entire operating system onto a USB thumbdrive, so you can boot Windows basically everywhere: at your home, at friend's home, at work, at public library or internet-cafe...
With FlashBoot, these dreams come true. FlashBoot can install fully-functional Windows 10/8.1/8 to USB thumbdrive...
Simply put, that's Untrue. I would have told folks earlier, but it took roughly 4.5 hours to find out -- the app's That Slow. Thankfully there isn't much to the install -- a program folder with a few files, Start Menu shortcuts, a uninstall key in the registry, & a key without any values [hence useless] for the app itself.
Basically FlashBoot does pretty much what rufus does [maybe a bit less], but with a wizard type GUI. I tested it's claims about the portable mini version of Windows. Such a thing is possible, often using somewhat universal driver packs & often using QEMU [wikipedia[.]org/wiki/QEMU]. Since most of FlashBoot's code is packaged in a single, large .dll file, couldn't easily tell if that stuff was in there or not, so I gave it a try. It's Not.
I pointed FlashBoot to the ISO for an Insider build of win10 Pro 64 bit, then showed it a new Kingston DataTraveler 100 G3 USB 3.0 32GB USB stick. 4 hours or so later it accomplished the same thing anyone can do using Windows DISM in less than an hour. The only benefit to FlashBoot is that it used so few resources that I had to use [Sysinternals] Process Explorer to tell if it was still running, so in that respect it's an ideal background app.
Anyway, Windows is delivered in an image file in a Microsoft format called .wim. When you get that image file via Windows Update it's in a compressed & encrypted image file called [usually Install].esd. To turn either into a set of installation files [e.g. an ISO], the code that checks compatibility & provides the windows with dialogs is added alongside that .wim or .esd. When you run setup.exe & go through the dialogs the install routine uses DISM to extract the Windows files & folders from the .wim or .esd & copies them to the target hard drive partition.
You can easily do the same thing yourself, running DISM from the command line, but to do it with a win10 .wim or .esd you have to be in 10 [you can do 7,8, or 10 from 10, 7 or 8 from 8 etc.]. And when you use DISM that way you need to separately add the boot files using BCDBoot [also included in Windows]. You can also run DISM to extract those files to a VHD [Virtual Hard Disk], make a disk image backup, & restore that backup wherever you want to install Windows.
However you do it you wind up maybe midway through the installation process, and Windows will set about finding out what drivers it needs & adding them, before it gets to the dialogs where you can supply your Microsoft account & so on. And because drivers for the device it's running on Are Added, the only way it could be portable is if every device you used it on was identical. That doesn't of course address activation & licensing issues.