Where can I find a real architecture document on WordPress?

I can only find some db schemas, what about all the rest of business logics in UML ?

I’m not aware of any besides michel fortin’s WP text flow diagram from a few years ago:

http://michelf.com/weblog/2005/wordpress-text-flow-vs-markdown/

That said, there were recent talks about writing some kind of plugin dev’s companion reference with the full flowchart of the WP hooks and API.