How to open an item in the center of the screen in iPadOS

In many iPad apps—including Mail, Messages, Notes, and Files—you can open an item in the center of the app window. To do this, do any of the following: 

° Touch and hold a message in a mailbox in Mail.

° Touch and hold a conversation in Messages.

° Touch and hold a note in Notes.

° Tap Open in New Window.

The item opens in the center of the screen, on top of what you’re viewing, without otherwise changing your view.
Tip: You can also pinch open any of the items listed in step 1 above to open them in a new window in the center of the screen.
You can change a center window to full screen by tapping 

Make it a Split View window by tapping

Or change it to a Slide Over window by tapping 

You can also use the App Switcher to switch between apps in Slide Over. 

(This how-to is based on my experiences and info on Apple’s support pages — where the images sometimes come from.)