Somehow I have lost my calendar app on my phone. How do I restore it from iCloud?

First, go to the App Library (the last page of your Home screen) and see if it is still there; you may have only removed it from your Home screen. If you find it drag it to wherever you want it. If it is actually gone, see→Delete built-in Apple apps from your iPhone, iPad, or Apple Watch - Apple Support

Specifically the section:

Restore a built-in app that you deleted

1. On your iPhone, iPad, or Apple Watch, go to the App Store.
2. Search for the app. Make sure that you use the exact name of the app.
3. Tap the Redownload button .
4. Wait for the app to restore, then open it from your Home Screen.

If you have Apple Watch, restoring an app to your iPhone also restores that app to your Apple Watch.