Built-in calculator

Yes, it does. Look in the App Library (the last page of the Home Screen) and, if you find it, drag it to the Home Screen.

Otherwise search for the app (pull down the Home Screen from the middle of the screen). If the app is not found, it probably was deleted. In which case, download it again: App Store - Calculator.

Yes. The OS includes a calculator app. This app can be deleted by you though or it may have been removed from the Home Screen and is still available in the App Library. Here are your options:

• Find it in the App Library - Find your apps in App Library on iPhone - Apple Support
• Redownload it from the App Store if it was deleted