A lot of things have changed in iOS/iPadOS 15.
There are close to 500 new features.
Many can be disabled. Many cannot.
There is NO way to know what features your father may like or may not like about iPadOS 15.
Here is a thorough YouTube video that covers most of the iPadOS 15 new features.
https://youtu.be/40nIyS6duRg
As far as your issue above.
Those are iPadOS widgets
This is the new way iPadOS 15 displays Widgets now.
Widgets cannot be turned off or disabled.
Widgets are flexible and dynamic, now.
You can move these around on any Home screen and can have multiples of the same widgets on any Home screen.
They can be sized differently in order to show more information or smaller to show less.
You cannot disable widgets.
So, if your father does not like these either, you may just want to move them to another blank Home page.
Use widgets on your iPad - Apple Support
You and your father will need to figure out what new features he may wish to keep and uae or disable.
There are too many to cover in a volunteer device/software user to user support community.
Good Luck to You!