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
Slide Over and Split Views remain - although are accessed differently. You can have two or three App windows side-by-side - or a spit the screen into four quadrants.
To access, assuming that the current App window is not maximised; touch-and-hold the traffic-light icon at top-left of the App window to expose the Fill & Arrange options:

NB: If you update to iPadOS 26.1, you also have a Slide Over view from which to choose:

Selecting a layout will cause the current App window to move aside, allowing you to open another App; the selected view will open the selected Apps in their own windows - which can be resized laterally, as before, by dragging the divider to left or right.
If the App window is already maximised, swipe downwards from the top-edge of the screen to expose the new menu bar - then use the traffic-light icon as described above.

Remember, for iPadOS 26, you have a configuration choice to use Full Screen Apps only, use Windowed Apps, or Stage Manager mode as you prefer:
Settings > Multitasking & Gestures - select your preferred multitasking mode

As for the Dock, there is now an additional setting that controls whether it remains visible or requires a swipe to appear on the Home Screens:
Settings > Multitasking & Gestures > [Gestures] Swipe to Reveal Menu Bar - set to ON or OFF as preferred