I don't know the location of any particularly good explanations, but I'd explain it as one part start-from-scratch distributed/federated computing environment, one part community rebuilding an ecosystem and new internet on top of that environment, and one part performance art by making everything different as if plucked from a parallel universe that diverged at the dawn of computing.

This is a pretty good one: https://www.hyperstition.al/post/urbit-an-introduction/

There's also a podcast called Understanding Urbit that's a decent introduction.