What stack is Windchill running?

Windchill is using many third-parties in addition to its specific apis

I’m excluding rest and info engine from this answer 

If you would like to know the Stack so that you learn them before you start working with windchill as a developer that would be very good from pedagogical perspective

Windchill is using spring MVC for its Client Architecture (UIs where JCADebug works)

+ Windchill is using its own Persistence Framework

To customize the client architecture, you must have knowledge of Java, jsp, expression language, jstl, spring core and spring mvc, then understand how windchill is using them since the client architecture is built on-top of these technologies

You will deal often with the client architecture as a developer, for domain classes you will deal with the persistence framework 

Events and listeners use multiple and complex patterns such as observer, transaction, etc

Spring core is not used in services, instead Windchill has its own service delegate mechanism (the mapping between interfaces and implementations)

A good knowledge of RMI is also required in some cases, plus the client data model for business reporting and persistence apis

Windchill introspection mechanism but you will not touch it in most cases

I believe the complexity come from the enterprise abstraction which is hard to understand