I've been a committer on Apereo CAS (https://github.com/apereo/cas) since the 3.0 days. It's an open-source SSO/identity platform - 400+ modules, Spring Boot 3.x, Java 21+. Deployed at thousands of universities, government agencies, and enterprises.

I wrote a book that walks through how the whole thing works internally - not configuration, but actual code paths. 30 chapters, 825 pages. The Spring Boot bootstrap sequence, the authentication pipeline, Webflow state machines, ticket registry internals, SAML2/OAuth2/OIDC protocol engines.