The architecture of MediaWiki
The Architecture of Open Source Applications: MediaWiki it's a very interesting read about the software that powers one of the busiest sites in the world, specially since it was specifically for Wikipedia. It's not terribly up-to-date, but it's a good introduction to understand how MediaWiki works in general, nevertheless.
https://felipe.lavin.blog/2022/05/17/the-architecture-of-mediawiki/