You have clearly managed to enable a Safari Slide Over view - this being a major multitasking feature of iPadOS, fully detailed in the iPad User Guide for iPadOS.
Here is the easy way to change the Slide Over view, to full screen... Identify the small grey drag-bar at top-centre of the Slide Over window. Touch-and-drag the drag-bar to the top-centre of the screen - and lift your finger; the Slide Over window will transform into a full screen view.
Here are links to both the online and downloadable versions of the User Guide:
https://support.apple.com/guide/ipad/welcome/ipados
https://help.apple.com/epub/ipad/en/
You may also find these Apple’s YouTube videos to be helpful in understanding basic operation of the Slide Over and Split View features of iPadOS:
https://youtu.be/ITzy5J3j5Is
https://youtu.be/nSBZKr5kXYM
You also have the nuclear option - that if disabling Multitasking:
Settings > Home Screen & Dock > Multitasking > Allow Multiple Apps - set to OFF
I hope you find these materials to be helpful in resolving any issues that you may be experiencing.