simulator
Like a pretend phone on your desk, it shows how the app will behave without needing a real device.
It is a model car for software. A simulator or emulator runs a virtual phone on your computer so you can test the app quickly.
It is fast, good for screenshots, and easy to reset. Some features need real hardware though, like camera quirks, push notifications, or in-app purchases.
On macOS, Xcode includes the iOS Simulator. For Android, Android Studio includes the Emulator. Expo can launch both from the terminal, and you can have several device sizes saved.