There's a lot of people involved, and a lot of various PR's and issues in github. As with most projects, this is where it's a situation that the game is so large and there are only so many people maintaining the project and so many people testing or confirming issues.
The project is a fork of a fork of a fork, for the most part. Much of the code comes from either SunwellCore (parent), or more likely TrinityCore (grand-parent), and over the past few years since the SC fork has been maintenance and bugfixes. I don't think any of the "current" contributors wrote the code that manages the actual game world, for example. My biggest complaint, I would say, is that at times there's a lot of "well this is how we've done it in the past" as opposition to a new feature or pull request
I think I would consider AzerothCore to be as complex as a medium-large monolithic service, I'd say. That's basically what it is, since the client is "off-the-shelf". There's a lot going on in the source code, but it's generally not difficult to figure out where an issue is.
I hope that helped answer your question.
