spikeyxxx Could you please explain why we need a second "Mapping Node" in order to stretch the "Volumetric Cloud" along "Local Y"?  Aren't  the scaling operations along the three axes independant from each other?

In the Livestream Kent mixes in a Noise Textures with the Texture Coordinates, which is a commonly used trick to distort the Coordinate system. However by using the Linear Light blend mode, he makes sure that the coordinate system is only distorted,  not 'shifted' as well.

If you look at an F-curve in the Graph Editor and add a Noise Modifier, using the default Replace Blend Mode, the Noise averages out to the original curve. This is what you'd expect and usually want. And this is what Linear Light does for you.  A bit like setting the Mid-level of a Displacement to 0.5. As Noise has an average value of 0.5, this gives a nice and controllable result.

If you do not use Linear Light in your MixRGB when mixing Noise with your Coordinates, the Origin (read: the whole coordinate system) will move (the Origin goes towards (0.5, 0.5, 0.5), depending om the Factor, the amount of Noise you mix in).