You need to explore and understand iPadOS multitasking - as this core feature cannot be turned-off.
You have a slide-over view of Safari enabled. Key to accessing and controlling multi-tasking is the small “three-dot” button that is found at the top-centre edge of the App Windows. Tap the three-dot button at the top of the Safari slide-over view; this will provide a menu - select the full screen button.
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