Getting rid of small side screens

You perhaps have a Slide-Over view enabled, this being a feature of iPadOS.

The easiest way to switch from Slide Over to a full-screen window is to touch-and-hold the small drag-bar at the top of the Slide Over Window, drag to the top-centre of the screen - then release; this window will become full screen.

Take a look at these Apple video guides:

After viewing, you’ll be ready for the additional Multitasking Guide:

https://support.apple.com/en-us/HT207582

The iPad User Guide for iPadOS is also a rich source of information. Here are links to both the downloadable and online versions of the Guide:

https://help.apple.com/epub/ipad/en/

https://support.apple.com/guide/ipad/welcome/ipados

I hope this information proves to be helpful in both explaining and resolving issues with the functionality that you have encountered.

Have it inspected at an Apple Store or Authorized Service Provider. After checking it out, they'll offer you all the available options.

Regards

Giulio