MichelPM,
I don’t know what you saw in my screenshot that would have you think I’m using iPadOS beta. I’d never even heard of that, except to read on this forum that it’s not discussed here.
Besides providing a screenshot of my open Widgets, I provided two Apple Support instructions on using them. Do you view them? Do you think I’d offer information to share to questioners with which I’ve had no personal experience?
I’ve always had the Calculator available on all of my iPads. While it’s new to me to use Widgets, that fact is nothing new, as I didn’t known how to use the Control Center, either, ‘till a recent class. But the calculator has always been available...
I’m using the ‘18 iPadPro, iOS 12.3.1, and that same calculator has always been available on my previous iPads, also.

