Multitasking is a feature of iPadOS for iPad - and is not an element of iOS for iPhone. Sidebars are feature of some Apps designed for both iPad and iPhone.
Unlike iPadOS14 and earlier versions, multi-tasking cannot now be disabled from settings. Multi-tasking has received a significant overhaul, both simplifying its use and introducing new controls. Whilst multitasking cannot be disabled at the OS level, you are not obliged to use any of its features.
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