Admittedly this is splitting hairs but
  1. Pads have not run iOS per. se. in a couple of years, instead using a distinct variant, iPadOS, that has more macOS-like features. Many iPadOS apps will run on Apple Silicon Macs and a small, but growing number of macOS apps will run on iPadOS.
  2. There is no Preview app or equivalent in either iOS or iPadOS, just as there is no Finder function. So there is no default app to open any file of any type.
  3. It isn't easy for long time macOS user's to fully get their head around what is going on in iPadOS and iOS.


Assunming you are running iPadOS 16 this may help your perception of what is going on on your iPad.
  1. In IpadOS 16 Go to Settings > Control Center and add Stage Manager to the list of included controls.
  2. Then swipe down from the upper right corner of the screen to reveal the Control Center functions
  3. Click on the Stage Manager icon. aa
  4. If and when you have more than one app open your screen will look something like this and the icons will indicate which apps are currently open and running and usually the with focus will have an identifier so you know what app you are in.
  5. If the foreground app is in full screen click and drag on the quarter moon icon at the bottom right of the window to reduce it.


By-the-way I use Stage Manager on both macOS and iPadOS most of the time.

If we knew what it was we were doing, it wouldn't be called research, would it?

— Albert Einstein