There's a hair / fur feature in Blender. Does that actually add geometry, or is it some form of after effect? What's the preferred way for adding hair to a mesh?

Are you referring to [1]? [2] is pretty recent and brief.

[1] Principled Hair BSDF https://docs.blender.org/manual/en/2.79/render/cycles/nodes/types/shaders/hair_principled.html [2] Tutorial - Game-Ready Hair Creation In Blender 2.8 (Hair Textures in Description) youtube.com/watch?v=W8oxZSft_Ns