Unlike iPhone, iOS/iPadOS for iPad has never included a calculator. There is some Apple folklore as to the reasoning, but as this is entirely irrelevant to your question, I’ll leave you to research this if it is of interest.
There are some good Ad-free examples of Calculator Apps obtainable from the App Store - and some very good paid examples.
One of the best free Calculator Apps for iPad, arguably far superior to the iPhone Calculator App, is Sci:Pro Calculator:
https://apps.apple.com/gb/app/sci-pro-calculator/id684978583
A paid alternative is PCalc:
https://apps.apple.com/gb/app/pcalc/id284666222