Emmanuel Lopez, a graphics programmer, writes breakdowns of how games and engines render frames.
HN links his 'A Macro View of Nanite', which a commenter offers as an explanation of Unreal's Nanite based on inspecting it in RenderDoc.
There is this now, someone trying to explain it after looking at it in RenderDoc:
http://www.elopezr.com/a-macro-view-of-nanite/
There's a little bit to be learned from the official documentation, too:
https://docs.unrealengine.com/5.0/en-US/RenderingFeatures/Nanite/