User Interface (UI) design is an art. With due experience, developers can sharpen their skills in this. Development tools (Xcode, Android Studio etc) provide lots of support to create the UI for the mobile apps
When customers need an app in multiple platforms (IOS, Android, Windows, web etc) our designers create one UI. Mobile platforms provide coding mechanisms to adapt the design to the screen sizes. For example the same design can be used in a phone as well as a tablet.