While avoiding needless repetition of the sage advice already offered by others - and focusing specifically upon your evident difficulties…
Multitasking
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.
Key to accessing and controlling (or simply avoiding) iPadOS 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
Restoring an earlier version of iPadOS
Unfortunately, rolling-back to an earlier version of iOS/iPadOS simply isn’t possible.
All System Updates are digitally signed by Apple - and can only be installed whilst the digital signatures remain valid. After release of a new iOS/iPadOS version, the immediately preceding version continues to be digitally signed by Apple - however, usually within a few days, Apple will revoke the digital certificate with which the earlier version is signed.
Perhaps consider that iOS/iPadOS updates include both functional and essential security updates. Restoring an earlier version of the OS would undermine the security of your device and your personal data.
Safari
Many users who complain of issues with multitasking features encounter their problems when using Safari - or more specifically, when attempting to enter or modify the URL or search within the search bar at top centre of the Safari window.
The Safari URL/Address field is centre-justified. While you can tap anywhere in this field to edit the website address, most users have an unconscious conditioned habit of tapping visible text in the centre of the field - often accidentally triggering the three dot (ellipsis) multitasking button at top centre of the screen (immediately above the address field).
To ameliorate this potential problem, instead of tapping the centre of the address field, instead try forming a new habit of tapping either side - anywhere within the field other than the centre; in so doing, you can completely avoid accidental activation of the “three dot” multitasking button. Similarly, this change permits you to easily tap any of your open tabs, including those towards the centre, without triggering multitasking options.
The Shelf
Another area of regular comment (or complaint) by those new to iPadOS15 is “the Shelf” - this being a new feature of the revised iPadOS multitasking environment.
iPadOS permits you to have multiple “instances” of the same App - each instance being self contained. The shelf, when triggered, shows a thumbnail of each running instance of an App.
For example, if you have three running instances of Safari, each with its own set of open tabs, you will see three thumbnails corresponding to each running instance of Safari - plus a button to open a new instance of Safari. Simply tapping a thumbnail will switch directly to that running instance of Safari. Each App - such as Mail, Safari, Notes, Calendar, etc. - has its own corresponding shelf.
Individual open instances of an App can be closed directly from the Shelf; just swipe the unwanted thumbnail off the top of the screen. Note that similar to other Multitasking features, associated Shelf functions cannot be disabled
This article from Appleinsider has a useful explanation of this new feature along with many of the updates to multitasking:
https://appleinsider.com/articles/21/08/12/how-to-master-multitasking-on-ipad-and-ipad-pro-in-ipados-15
The iPad User Guide is also a rich source of frequently overlooked information. 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
I hope that this insight proves to be helpful in both understanding - or perhaps successfully avoiding - the multitasking features of iPadOS15.