Learning iPad for non-computer literate seniors

I would start with the user guide.

click here ➜ iPad User Guide - Apple Support

Also, YouTube videos can be helpful. Here's a playlist of iPad usage videos for Seniors.

https://www.youtube.com/playlist?list=PL6CRpzAdjBZmC0NTpASMPuQ-LGLGgolNu

The iPad User Guide is a rich source of frequently overlooked information about your iPad and iPadOS. The Guide is online using a web browser (such as Safari):

iPad User Guide - Apple Support

https://books.apple.com/book/id1567104892

The Tips App, installed by default on all new models of iPad, provides a brief introduction - and leads you to the User Guide. In case you missed it, here is a link to the Tips App:

https://apps.apple.com/gb/app/tips/id1069509450

Apple also has a huge YouTube Channel - containing videos and feature guidance:

https://www.youtube.com/user/Apple

There are also comprehensive guides for Apple’s main productivity Apps, such as Numbers (spreadsheets), Pages (documents) and Keynote (presentations):

Numbers User Guide for iPad - Apple Support

Pages User Guide for iPad - Apple Support

Keynote User Guide for iPad - Apple Support

Another User Guide that may be of interest to you is the iCloud User Guide:

iCloud User Guide - Apple Support

When using and learning about Apple devices and features, you will almost certainly encounter iCloud services.