Can we swap the contents between two screens?

[hajime]

What key combination to use?

 

[AZhappyjack]

AFAIK, you cannot "swap" screen content with a key combo... you have to drag the items from one screen to the other to effect what you want to happen.

 

[hajime]

Is there a 3rd party app that can do that?

 

[MacGizmo]

Is there a 3rd party app that can do that?

Rectangle (Free) can do that easily with key combos. It also allows you to resize windows with shortcuts and snap windows to sides & corners of screens.