How do I reinstall the calculator widget to my iPhone home screen?

If it was deleted from the phone, see the following from Delete built-in Apple apps on your iOS 12, iOS 13, or iPadOS device or Apple Watch - Apple Support

Restore a built-in app that you deleted

1. On your iOS or iPadOS device, go to the App Store.
2. Search for the app. Make sure that you use the exact name of the app. Find the correct name of built-in apps.
3. Tap  to restore the app.
4. Wait for the app to restore, then open it from your Home screen.

or if it is still on the phone and you just want to move it, see: How to move apps and create folders on your iPhone, iPad, or iPod touch - Apple Support