How can I reinstall the Siri app?

Siri is a service built-in into iOS, watchOS, macOS, and HomePod, and you cannot download it through an app. If you cannot use Siri, you might have disabled Siri, or never activated it. To check if it is enabled, hold the home button down for a second if you have an iPhone 8 or earlier, and hold down the Side Button for a second if you have an iPhone X. If nothing happens, then Siri has been disabled. To enable Siri, go to Settings > Siri & Search, and turn on the switch where it says 'Press Side Button for Siri'. Have a great day 🙂

In addition to what FrankyFG correctly suggested, follow also the steps provided in this support guide:

Get help with Siri on your iPhone, iPad, or iPod touch - Apple Support

and see if you can have Siri back on your device.

Regards

Giulio