You need to explore multitasking - as you appear to have an slide-over view of Safari active on your iPad. Key to accessing and controlling multi-tasking in iPadOS15 is the small “three-dot” button that is found at the top-centre edge of both the main Safari window - and the slide over view.
More information about using multi-tasking and controlling individual windows 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