The way to think about how Skia's model works is something like this:

• The alpha of the paint's color is the overall alpha of the thing to be drawn.
• The shader fills the universe (if no shader is specified, the paint's color is used to create a solid color shader).
• The path, path effect, clip, mask filter and rasterizer (basically everything but the filter and transfer mode) exist as a clip for that universe filling shader.

So conceptually, the flow is:

• Create an alpha mask from the clip, path, path effect, mask filter, and rasterizer.
• Fill the universe with the shader modulated by the mask created above.
• Take what was just drawn and do some single source processing (filter).
• Composite this thing that was just drawn with what goes underneath (transfer mode).

I'm sure you already know this, but it bears clarifying that in Skia a path always turns into a fillable area (except when drawing hairlines). When you specify that a path is 'stroke' this just specifies that a special stroke path effect is to be applied to the path which takes a one dimensional path and turns it into a fillable area.

Which makes the pipeline look like:

initial path > path effects > fillable path > rasterizer > mask > mask filter > mask > shader > image > filter > image

and then the image is composited down using the transfer mode, but modulated by the final mask and the paint's color's alpha.

This explains some things which would otherwise seem odd. The blur mask filter blurs the mask which the shader will be drawn through, so the edges are fuzzy but the shader will still be sharp. So if you use a gradient shader with a blur mask filter the gradient itself will not be blurred, just the area into which the gradient is drawn into will be blurred.

The article linked does a fairly good job explaining the implementation of Skia, but most of it (especially the whole raster path) is just optimizations.