They don't: there are different UIs for different screen sizes.
Your data is in the cloud (eg a Word Document in OneDrive on on Azure) and you can access it from a full desktop program on a PC or Mac, from an app on iOS or Android, or via a web browser on any device that has one.
This is a cross-platform vision that works both online and offline (unlike Google) and that ultimately doesn't care which device you use (unlike Apple).
But that's not the vision from the article ("Microsoft is about to turn a phone into a real PC") or this thread.
Take a phone and plug in a bigger display and keyboard and mouse and suddenly it's a desktop computer. So a single application is going to need to cope with a 4" touchscreen display with no keyboard or mouse as well as a multi-monitor setup without touch, but with a keyboard and mouse.
> So a single application is going to need to cope with a 4" touchscreen display with no keyboard or mouse as well as a multi-monitor setup without touch, but with a keyboard and mouse.
Yes, it reflows. You use the app when it's in phone mode, and you get the full desktop-style UI when in Continuum mode. There are several demos online. See for example, from April 2015 (so it's surprising you haven't seen it before):
> So a single application is going to need to cope with a 4" touchscreen display with no keyboard or mouse as well as a multi-monitor setup without touch, but with a keyboard and mouse.
Only if it makes sense for that application. It doesn't mean that every desktop app would be usable on the phone. Or vice versa, for that matter; although it is much more rare for a phone app to not be useful on the desktop.
As a desktop user, Windows 8 seemed ludicrous to me. Fire up a 4-function calculator or start the weather app and it gave me a full-screen view. Windows 10 is far better although they still show UI elements sized for touch even though I don't have a touchscreen.
> It doesn't mean that every desktop app would be usable on the phone.
The base computer should probably be a watch, not the phone.
I think the baseline depends on the type of app. For something like an alarm clock or calculator, yeah, it should probably be a watch. For something like maps, a phone makes more sense. For an office suite, I would expect the phone to have full viewing functionality, but editing should probably require at least a tablet (not to say disable it on the phone... just that it doesn't matter much if UI is awkward). Something like an IDE? Laptop/desktop, with mouse-centric UI.
Your data is in the cloud (eg a Word Document in OneDrive on on Azure) and you can access it from a full desktop program on a PC or Mac, from an app on iOS or Android, or via a web browser on any device that has one.
This is a cross-platform vision that works both online and offline (unlike Google) and that ultimately doesn't care which device you use (unlike Apple).