Basically, there’s no difference in how you can arrange iPad app windows in iPadOS 15. However, it’s more convenient working with windows.
For example, while you’re using an app, you can switch it to a Slide Over window—a smaller window that slides in front of another app or window—and open another app behind it. For example, while you’re using the Photos app, you can have Messages open in a Slide Over window and carry on a conversation while looking at photos.
iPad keeps track of the apps you open in Slide Over so that you can easily switch between them.
To open an app in Slide Over
° While using an app, tap the three button icon at the top of the screen, then tap the center button on the multitasking icon.
° The app you’re using moves to the side to reveal your Home Screen and Dock.
° Find and open the app you want to appear behind the Slide Over window.
° The second app opens, and the first app appears in a Slide Over window in front of it.
To open a third app in Slide Over when the screen is in Split View (on supported models), swipe up from the bottom edge just far enough to reveal the Dock, then drag the third app from the Dock to the Split View divider.
To switch between apps in Slide Over
Swipe right along the bottom of the Slide Over window, or do the following:
° Swipe halfway up the screen from the bottom of the Slide Over window, pause, then lift your finger.
° All the Slide Over windows appear.
° Tap the app you want to view, if it’s visible.
If you don’t see it, swipe left and right through the apps.
To move the Slide Over window
° Move the Slide Over window to the other side of the screen: Drag from the three dot icon at the top of the Slide Over window.
° Temporarily hide the Slide Over window: Swipe all the way up from the bottom of the Slide Over window, or swipe the three dot icon or drag either side of the window to the left edge of the screen. The Slide Over window disappears and a tab appears to indicate the Slide Over window is still available.
° Move the Slide Over window back onto the screen: Drag the tab indicating the Slide Over window from the left edge of the screen.
(This how-to is based on my experiences and info on Apple’s support pages.)