Unlike iPadOS14 and earlier versions, multi-tasking cannot now be disabled from settings. Multi-tasking is now integral to iPadOS and has received significant overhaul in iPadOS15/16 - both simplifying its use and introducing new features and controls.
Key to accessing and controlling multi-tasking is the small “three-dot” button that is found at the top-centre edge of the screen.
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