Figma Layer Icon Meaning

a) what this icon means
Vector Layer

b) why it sometimes appears (rather than a vector shape icon)
By default, Figma would make a mini icon of the vector you created. But my guess is if it’s too complex like it’s a single path and there’s plenty of holes overlapping each other (most of the time for non-stroke vectors), it will just generate that icon

c) why outlining strokes doesn’t work with it
Stroked icons are very easy to code since they’re just a bunch of connected points, but for outlined stroke it’s more complicated especially if there’s plenty of holes

d) if you can convert it back to a vector shape
I’m not sure. I’m also not sure about what you mean. It’s a vector though