I think the structure is very simple. It's just a lot of items like your comment is item 41207393 as in https://news.ycombinator.com/item?id=41207393

I think that is just written to disk as something like file41207393 when you click reply.

When the system needs an item it sees if it's cached in memeory and otherwise reads it from disk and I think that is pretty much the whole memory system. Some other stuff like user id that works in the same sort of way.