You've got a few responses spelling out a few ways it can be done.
Almost all of them are based off something called "WinPE" or the Windows Preinstallation Environment. [0]
This is a similar OS, used when installing your OS. It usually shares the same kernel, but with everything stripped away, including useful things like the desktop, etc. You can add some, but not all, of these things back.
As WinPE uses the Fat32 file system by default (you can change it with some difficulty), most of these installations will be less than 4GB.
Building a WinPE-based environment is a fun weekend project. Totally against the license agreement to make it into a useable OS, but that shouldn't matter if you never share it and are just using it to get to grips with how WinPE works.
Microsoft also today has containers and images that they themselves streamline as much as possible, for certain use cases. The two most common I've seen referenced:
1. Windows IoT Core [0] designed for IoT devices and embedded scenarios. The RPi build of Windows 10 IoT Core is around 800 MBs. (The Raspberry Pi build is an interesting one to note because you can explore yourself easily in a weekend project.)
2. Windows Nano Server [1] is a fascinating Windows 10 build intended for among other things Docker containerization [2] and virtualization projects. I've heard some images of Nano server are as small as about 400 MBs. Given its intent for containerization/virtualization, it's pretty amenable to experimenting with in weekend projects, too.
Microsoft has seemed quite interested in the last few years with how small and modular they can make Windows under the hood. It's not always apparent at the consumer level because of backward compatibility and all of the required "modules" (like Win32) that folks think of "as Windows".
Almost all of them are based off something called "WinPE" or the Windows Preinstallation Environment. [0]
This is a similar OS, used when installing your OS. It usually shares the same kernel, but with everything stripped away, including useful things like the desktop, etc. You can add some, but not all, of these things back.
As WinPE uses the Fat32 file system by default (you can change it with some difficulty), most of these installations will be less than 4GB.
Building a WinPE-based environment is a fun weekend project. Totally against the license agreement to make it into a useable OS, but that shouldn't matter if you never share it and are just using it to get to grips with how WinPE works.
[0] https://docs.microsoft.com/en-us/windows-hardware/manufactur...