I read this article earlier today and ended up going down a rabbit hole trying to figure out what Xanadu actually "is" (on a more technical level) but haven't really been able to find much. Does anyone have a good resource for understanding the details?

Think of it as distributed source control on steroids. There's a document, built up by changes. Each change is owned by somebody. To read the document, you have to pay everybody who contributed a change. (That part was always kind of hazy, but there were lots of microtransactions involved.)

You can link between documents, too, like the web, and all links are multi-ended, so you can see who referred to what.

All of this is kept consistent, like an SQL database with atomic transactions.

The original documents are confusing because the concepts needed to build Xanadu didn't really exist then. Nelson also introduced new terminology never used elsewhere, and mostly not implemented. Now we have consistent databases, CRDTs, links, and the Web as a model, plus lots of cheap bandwidth and storage. So we know how to talk about and build this stuff.

And a whole new era of questions. Who owns the weights when an LLM is trained on Xanadu?

You could build it, but no one would come.

A lot of details haven't been explained clearly, as far as I know, but the basic idea is in Nelson's books.

There's no embedded markup. Links and formatting is off to the side in other documents, referencing locations in the original document.

You also have "transclusions," where you say "in document A position A.n, embed part of document B from position B.n to position B.p."

By using transclusions, you can make new versions of documents, without messing up the links and formatting codes even though they aren't embedded and reference by character position. Because you're not editing any original documents, anyone can effectively make changes. You can add a link to a page without even making a new version, you just put a new link in a side document.