why is the smoke "transparent" when we use an add node

The "add" operation is a mathematical way of combining images together. Think of each pixel in the image as a number value (black and white for the sake of explanation):

• A pixel on image 1 (bottom image) is grey, so a value of 0.5.
• The corresponding pixel on image 2 (top image) is a darker grey, a value of 0.25.
• When you ADD them together you get a lighter grey value of 0.75

I should add that while a value of 1 (white) is opaque, the closer the value gets to 0 (black) the more transparent its effect on the image(s) below. Black would have no effect on the image(s) below.

Lol one more thing. The "Add" operation is often used to push pixel values above 1 for a brightening effect. So adding a 0.75 grey pixel value to a 0.5 pixel value takes you above 1: to 1.25. Commonly, your computer's display has to normalize that value back to a 0-1 range and the result is an over-exposure effect.

Think a bout a lightsaber: The middle of the beam is white representing an above-1 overexposure. But as the beam fades outward it gradually becomes a color (like red for the bad guys). The fade into color represents pixel values that are gradually dropping below one and capable of showing color.