How to control an Ipad Pro 2020 via internet from my Desktop Mac (in order to easily assist my grandmother)?:

Whilst remote access and support solutions of other computer systems can be readily found, the iOS/iPadOS security architecture does not permit remote control of an iPad.

Sharing/broadcasting your local screen to another computer does not constitute remote control. TeamViewer for iPad (suggested by Michael Black above) - and other available Apps - do not provide remote control of an iPad.

I am sorry that this reality is not what you are hoping to learn.

You could look into TeamViewer QuickSupport (https://apps.apple.com/us/app/teamviewer-quicksupport/id661649585)

It would allow her to share her screen with you, but I am not sure of just what it’s capabilities are fully with iOS or iPadOS.

Note that if the device has your AppleID on it, she could make her own purchases on your account with it, or even edit your password. I’m not saying she’d do it deliberately, but she can, inadvertently, do anything to your AppleID and iCloud accounts that you can do.

It’s suggest she have her own AppleID and iCloud account and you can always set up family sharing if she doesn’t want to put payment info on her AppleID.

iOS/iPadOS has no facility for remote control or remote management of the device.  Similarly, there are no third-party Apps that provide remote control functionality.

The best that you can likely achieve is to use FaceTime (or other communication platform) - and literally “talk-though” a problem; if you have similar hardware, and the same version of iPadOS installed, you should be able to work through a problem step-by-step.