Design here covers the whole visible chain: the icon somebody sees in a search result, the screenshots that decide whether they download at all, the first screen after install, and the last settings row nobody thinks about. One language across all of it, so the app reads as one thing rather than as screens made in different months.
We design against the device, not against a canvas. A layout that looks composed at full size on a monitor still has to hold up on a small phone, in one hand, in the dark, at the largest Dynamic Type setting. That check happens while the design is being made, because the alternative is discovering it after the screen has been built.
We can deliver a design system for your engineers to build against, or implement the screens ourselves — either way the design gets judged in the running app rather than approved as a picture of one.