You need to explore iPadOS multitasking - as you have a slide-over view of an App enabled. Key to accessing and controlling (or avoiding) multi-tasking is the small “three-dot” button that is found at the top-centre edge of App windows.
You can close a slide-over view, or switch to split or full screen view, by simply tapping the three-dot multitasking button at top-centre of the associated App window. From the menu, select Close, or an alternative view.
More information about using multi-tasking can be found in this support page:
Use multitasking on your iPad - Apple Support
The iPad User Guide is also a rich source of information about using iPad and its many features. The Guide is available both online using a web browser (such as Safari), or as an Apple Books download:
iPad User Guide - Apple Support
https://books.apple.com/book/id1567104892
Unlike iPadOS14 and earlier versions, multi-tasking (including split-screen and slide-over views) cannot now be disabled from settings. Multi-tasking has received a significant overhaul with iPadOS15/16, both simplifying its use and introducing new controls.
Apple does invite submission of comments and feature requests via its Product Feedback portal. For iPad and iPadOS:
Feedback - iPad - Apple
If you would like to see greater flexibility in enabling or disabling multitasking features in a future version of iPadOS, this would be the most appropriate channel to make such a request.