To recap...
A SKM file is a Sketchup Material definition file.
A SKM file is a ZIP archive that just has a different file-extension.
Changing its extension to ZIP allows you to see what it contains - but the only thing you might be interested in is the res/Image file, which is there if the Material has a Texture; the rest of the contents are things like the Material's thumbnail used in the Browser and XML files that contain the Material's data - like Image/Size [if any], RGB, Opacity etc...
Placing a SKM file [unchanged] into a subfolder that is within the Materials folder [or any other folder that is in the Material Browser's search paths...] allows the Material that it defines to be displayed in the Materials Browser when that subfolder is chosen [like any other Material]; when applied to objects in the model that Material is added to the Model's internal Materials collection.
If you modify or newly create some Materials in a Model, then the Material Browser's pop-out/context menu options allow you to Save the Material[s] to external SKM files [as a 'collection' etc]: this allows you to 'make' a custom Material in a Model, then make a SKM version of it that is reusable in any other Model later on...
You do not need to 'open' a SKM file, just put it in a subfolder that the Materials Browser can see and it's automatically displayed - like any other Material...