Where is the calculator?

There are some features and native Apps that are only available for iOS (iPhone) or iPadOS (iPad) - the Calculator App being just one of the Apps that are exclusive to iPhone.

While the native Calculator App is not available for iPad, there are many free and paid third-party alternatives. One of the very best calculators in the App Store - arguably superior to the Apple Calculator, is completely free and without advertisements - is Sci:Pro Calculator:

https://apps.apple.com/gb/app/sci-pro-calculator/id684978583

The calculator app, as of the latest update, is not present on iPad. Use a product with the calculator app to use the calculator app. I recommend using a website alternative.

Hope this helps!