On iPad, iOS reserves the Command-Tab keyboard shortcut for switching apps on your device, which prevents its use on the remote Mac you're connected to.
Since apps cannot override this behavior, Screens provides some options:
On your iPad
Permute Command ⌘ and Control ⌃ keys
Enabling this option will allow the Control key to act as the Command key and vice versa. This way, you can use Control-Tab to switch apps on the remote Mac.
Switch apps with Command-\ instead of Command-Tab
If you prefer to keep the modifier keys unchanged, you can use Command-\ to switch apps on the remote Mac. This workaround also bypasses the iOS limitation with just a single keyboard change.
You can find both options in the app settings on your iPad:
On your Mac
You can also permute Command and Control keys when using Screens on your Mac:
