Learn English – difference between “to air” and “to broadcast”

The sky is the blue (or black, or cloudy) vault that comprises the visual limit of the eye. Things that appear to be above you such that that vault appears as their backdrop are said to be "in the sky." The tops of skyscrapers appear to be "in the sky" as do birds and airplanes.

But an airplane taking off or a bird flitting from a low branch to the ground is moving through the air, but is not in the sky. A fly ball in baseball may appear to be sky-high momentarily, but eventually comes to earth. It's all about perspective and point of view.

Swap means "exchange", while switch means "change".

When you swap something, you replace something with something else:

house swap, wife swap

When you switch something, you move from something to something else:

Switch language, switch flight