The Apple wallet App is designed to facilitate easy contactless payments using NFC - including mass transit transactions. iPad lacks NFC functionality as it does not incorporate the necessary hardware components - nor is it practical for this purpose due to its relative bulk.
Recent models of iPad do, however, support online payments via ApplePay.
Not all Apple Apps are universally available for both iPhone and iPad - some are only available on one or other platforms. In respect of the iPhone Calculator App, this is just one example of an Apple App that does not have a corresponding App on iPad. There are, however, many third-party Calculator Apps in the Apple App Store. Some are free - and some are actually better than the iPhone Calculator App.
Here is one example that is both free - and arguably better than Apple’s offering:
https://apps.apple.com/gb/app/sci-pro-calculator/id684978583