A folding phone, with both screens under your control
The iPhone Duo template moves through the open display, hinge, cover and back. Use a wide screenshot for the inner display and a portrait screenshot for the outer cover.
In the editor, select the device and open Media → 2 screens. The numbered Open display and Outer cover cards each have their own Replace and Crop controls. One wide image spans the inner display; the cover can show a different screen.
For a single-screen app, start with an iPhone mockup. For a website or dashboard, choose a MacBook mockup with a screen hold that leaves the interface readable.