Each of the galaxies in the pack are rendered using a volumetric (ray marcher) shader effect, which is geared towards how galaxies are "normally" composited, along with the help of a texture which defines its inner structure. If you open the texture up in an image editor, and separate by color channel, you'll find that each of the channels in the texture have very different looks.

Red - Absorption

This channel defines most of the visual structure of the galaxy, along with the green channel, as the red channel amplitude defines the amount of absorption in any given point across the galactic plane. For any given point in the volume that the ray samples, absorption is a measure of how much light is absorbed from whatever is behind. Absorption rate can be thought of as opacity of the cloud. i.e.density of material that is non-emissive.

Green - Luminance

This channel defines how much the gas clouds will glow in any given location in the cloud, as measured perpendicular to the galactic plane. It largely affects the visual structure, along with the red channel, by defining how much light the gas emits at any given point. The green and red channels work in unison to create most of the visual structure of the galaxy.

Blue - Density

This channel defines the density of the galaxy. It will be weighted with a double bell function across the galactic plane and affects the "thickness" of the galaxy as well as the amount of gas that is present far away from the galactic plane.

Alpha - Transparency

This channel currently only has the effect of weighing against the density value from the blue channel, in order to balance emission levels at a high exposure setting. It can largely be ignored or derived from the other color channel values in your image editor.