Ever since Windows 8 was first shown to the public back in June 2011 people have speculated about / been confused about what relation the "new system" has to the existing Windows, technically - is "Metro" a thin layer over "Desktop" Windows, like Media Center or Bob? Or is it the reverse, the desktop some virtual-machine environment on top of the "new system", like Mac OS "classic mode"? Or are they two systems running side-by-side?
In fact it's none of those things - the new environment/UI isn't a new shell running on top of, underneath, or alongside the existing shell. Rather, it's a set of new features added and refactorings made to every layer of the existing system, together with a set of policies designed to create the effect of a fresh new environment by encouraging/enforcing a path through only the "new" parts of each layer.
So, for example, the app permissions / sandbox that new-style apps run in is actually a new general low-level system feature that desktop apps can use too - IE10 desktop uses it for "Enhanced Protected Mode". But this isn't really being publicized or promoted because why would desktop app developers rewrite their code to work with it? Similarly the windows that new-style apps run in aren't being controlled by some separate window manager, they are just regular windows (HWND) in the existing window manager - but that window manager has new features for smoother composition/animation and touch manipulation of windows, and new security features to enforce Z-order rules so apps can't pop up windows on top of new-style apps or steal focus or whatever. Most of the new WinRT APIs aren't available to desktop apps but there is no fundamental purely technical reason they can't be. And so on.
In fact it's none of those things - the new environment/UI isn't a new shell running on top of, underneath, or alongside the existing shell. Rather, it's a set of new features added and refactorings made to every layer of the existing system, together with a set of policies designed to create the effect of a fresh new environment by encouraging/enforcing a path through only the "new" parts of each layer.
So, for example, the app permissions / sandbox that new-style apps run in is actually a new general low-level system feature that desktop apps can use too - IE10 desktop uses it for "Enhanced Protected Mode". But this isn't really being publicized or promoted because why would desktop app developers rewrite their code to work with it? Similarly the windows that new-style apps run in aren't being controlled by some separate window manager, they are just regular windows (HWND) in the existing window manager - but that window manager has new features for smoother composition/animation and touch manipulation of windows, and new security features to enforce Z-order rules so apps can't pop up windows on top of new-style apps or steal focus or whatever. Most of the new WinRT APIs aren't available to desktop apps but there is no fundamental purely technical reason they can't be. And so on.