The scaling starts with keeping the screen ratio consistent between devices, even if that means going with a rather wonky resolution. The iPhone 5s has a resolution of 1136×640, which is 16:9 just like a TV. The 4.7-inch iPhone 6 clocks in at 1334×750, which is also 16:9. The bigger 5.5-inch iPhone 6 Plus has a 1920 x 1080 LCD panel. That, of course, is the same 16:9 screen ratio. That means apps can automatically scale up to the larger, higher resolution screens without breaking the layouts.
This is definitely a good thing, but the content of apps might still be blurry if developers haven’t updated the resources used. A button or icon designed for a sub-720p screen is going to be fuzzy on a 1080p screen–there’s no way around that. Text should render just fine as long as things have been done according to the rules. This is basically a step toward resolution-independent apps for Apple, but that’s something Android has been doing for a long time. On Android, developers can include multiple resources and the OS will use the one that matches the device’s DPI. Apps can also scale to different screen ratios without breaking.
Apple also opted to take full advantage of the iPhone 6 Plus’ larger display by making some apps dual-pane when in landscape mode. For example, your messaging app can show the contact list on the left, and currently active conversation on the right. This is something developers could use to great effect in a new generation of apps built for the 6 Plus. I suspect we’ll soon see changelogs that call out specific support for that phone’s larger screen.
Apple isn’t the first to come up with the dual-pane thing — Android got there back in 3.0 Honeycomb in 2011. Google calls this the Fragments API, and it allows developers to specify multiple modular UI elements within a single application UI. It’s very much like what Apple is doing with the iPhone 6 Plus. An app might be single pane in portrait, but switches to dual-pane in landscape. Fragments are also used to rearrange app elements on devices of different sizes.
Developers should have an easier time getting prepped for the iPhone 6 thanks to app scaling. Some apps will just work without changes, and higher-resolution resources should take care of many others. On the user side, you’re going to be seeing more functional apps the instant you take the iPhone 6 out of the box.