iPhone doesn’t support the multitasking interface that you’ll find in iPadOS.
Unlike iPadOS14 and earlier versions, multi-tasking cannot now be disabled (or explicitly enabled) from iPad settings. Multi-tasking has received a significant overhaul, both simplifying its use and introducing new 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