Start here with the excellent video intro:
Multitask on iPad with iPadOS 26 - Apple Support
iPadOS multitasking has received a significant makeover from that to which you may be accustomed in iPadOS 18.x and earlier - and may take a little time to learn and adjust your workflows. You would be well advised to review the official iPadOS User Guide and the many available online tutorials (such as YouTube) to become better acquainted with the new multitasking interface and associated gestures and controls.
Work with multiple windows at once on iPad - Apple Support
OS - iPadOS 26 - Apple
The three-dot multitasking button with which you are no doubt familiar from earlier versions of iPadOS has been replaced with the traffic-light button to access and control the revised window-based User Interface.

If you dislike the windowed environment, you can switch to using the basic Full-Screen App mode - but this disables all multitasking capabilities and hide the traffic-light controls; you will not be able to use split screen or slide-over modes with which you are likely familiar:
Settings > Multitasking & Gestures - select Full Screen Apps
