How do I install a free calculator for iPad Pro?

The iTunes and App stores, require a valid payment method, unless you created your Apple ID in a special way: Create or use an Apple ID without a payment method - Apple Support

If you didn’t, then you will need to provide a valid payment method on your Apple ID to access the stores, however, once verified, you are free to remove the payment method if you don’t want to keep it there, and will still retain access to the stores.

Change, add, or remove Apple ID payment methods - Apple Support

Using a gift card will not waive the need for a valid payment method.

Why do I have to put my credit card details in, to download a free calculator?

Because not all apps are free and some apps that are

free to download have in-app purchases.

If available in your location, you can also purchase an

iTunes gift card in various amounts and redeem that

to your account as a workaround.

As with all apps, you would download it from the App Store.

Does that mean that I cannot downloaded any Totally Free Apps on my new IPad???