Apple says the inner display is 50 percent larger than the iPhone 18 Pro Max screen, and because both screens share an aspect ratio, apps scale from the outer display to the inner one so users can pick up where they left off.
Apps built for the device will carry a "Made for iPhone Duo" badge on the App Store, where new editorial collections also go live Friday.
Netflix lets members browse Clips on the outer display and keep watching on the inner one.
Prime Video shows video on the top half of a partially folded screen with cast details below, while MLB pairs a live game with real-time stats.
Split View, which lets two apps run side by side on an iPhone for the first time, is behind Microsoft Word and Outlook layouts arriving in the coming weeks, along with Zoom, Slack and Fantastical.
Apple says the A20 Pro chip and a custom vapor chamber give the phone up to 35 percent better sustained performance than iPhone 17 Pro, which suits longer gaming sessions in titles such as Asphalt Legends.
Detail, a video editing app, uses Apple's new Core AI framework to run AI models on the device itself, which its CEO says keeps captions and audio cleanup fast and private.
Apple says more than 2,000 developers have attended over 50 in-person workshops in 19 cities, with more planned in London, Mexico City, Montreal, Paris and Toronto. Apps built with standard SwiftUI and UIKit components adapt automatically to each display, and the iOS 27.1 SDK adds layout, hinge-angle, scene accessory and camera tools, including support for the first under-display camera on an iPhone. Xcode 27.1 includes Device Hub, which simulates opening, folding and rotating the phone for testing. Apple says more app updates will arrive through the year, such as DoorDash delivery tracking, Zenless Zone Zero next month and the game UnGodly in November. iPhone Duo can be pre-ordered in more than 70 countries and regions, with availability in 28 more beginning October 30.