restart_app uses different mechanisms because Flutter targets do not expose the same restart primitives.
PlatformMechanismLimitations
AndroidRelaunches the main activity through PackageManager. Supports Android TV and Fire TV through a leanback launcher fallback. RestartMode.process and forceKill: true kill the process after launch for a clean cold start.None
iOSRecommended: opt-in Flutter engine restart that creates a new FlutterEngine, runs Dart again, re-registers plugins, and replaces the root FlutterViewController in the same iOS process. Legacy: local notification, exit(0), and user tap.iOS has no public API for automatic full process restart. Engine restart is not a process restart and cannot reset native singleton state. Legacy fallback requires notification permission and user action.
WebReloads the page using window.location.None
macOSLaunches a new instance through NSWorkspace and terminates the current process.Sandboxed Mac App Store builds cannot launch new instances of themselves. The package returns a failed result in this case.
LinuxReplaces the current process through execv.None
WindowsLaunches a new instance through CreateProcess and terminates the current process.MSIX-packaged Microsoft Store apps cannot be relaunched through CreateProcess.

​
Choose a mode

Use RestartMode.platformDefault unless your app has a reason to request a specific behavior. 
await Restart.restartApp(mode: RestartMode.platformDefault);
Use RestartMode.process when you explicitly want a full process relaunch on platforms that support it. Use RestartMode.flutterEngine on iOS when the host app has configured engine restart. Use RestartMode.notificationFallback only when the iOS user-tap relaunch tradeoff is acceptable.