SiriL Image Processing.

Being completely new to this Astro part of my photographic hobby, is there somewhere where I can find out exactly what all the “Image Processing” options actually do to an image, in plain English please?
So far I am using:
Colour Calibration
Remove Green Noise
Histogram Transformation (don’t really understand this)
Asinh Transformation (don’t really understand this either)
Background Extraction
Colour Saturation

But they’re only six of the 18 available?

Hello.
First I think you use a old version of Siril. You should update.
Second, the tool in the menu are disabled when sequence is loaded and not single image.

You may find a list of usual operations here: https://siril.org/faq/#in-which-order-should-image-processing-functions-be-used

You may find other information in the documentation, but it is being updated, so not everything is in it yet.