iPadOS 15: how to use the Shelf, how to find background windows0
But hey, at least now we have the Shelf — a new way to open and cycle through background windows for the same app. Note — this is not tab management, a-la Safari tabs, this is actual multiple instances of the same apps.
To make use of the Shelf, we need an app that has an "open in new window" option in their long-tap menu. At launch, Apple's core apps like Mail and Notes support this, but there's no reason to believe the option isn't available for 3rd party developers.
Anyway, we will use Notes for the example here. Just tap and hold on the note that you wish to open in a separate instance of Notes — then tap "Open in New Window".
The note now pops up in a floating window, on top of the Notes app. You can grab it by the tripe-dot menu on top and swipe it to the left or right of the screen, or swipe down to directly put it in the Shelf.
Now, the note stays open in the background — in a separate instance of the Notes app.
If you want to access the shelf at any time, tap the triple-dot button found at the top-center of your screen. The Shelf pops up from the bottom and you can see what windows are stored in the background.
From here, you can tap one of the windows to maximize it, drag it as a window over the main app on the screen, or swipe up on them to close them.
It's also worth noting that the multiple windows of an app can be found in your Recent Apps menu, the old-fashioned way.