The GNU Emacs Architecture: Unlocking the Core is a recent dissertation that introduces the internals of Emacs and how it works, with a focus on the latest concurrently features.
The site is often overloaded and inaccessible, so be sure to save the document once you finally open it.
https://www.diva-portal.org/smash/get/diva2:2052282/FULLTEXT01.pdf
This site has much better availability: