The MapStruct code base is thoroughly documented via JavaDoc. You can find the latest docs here. The individual pages contain generated class diagrams showing type hierarchies and package contents.
To get yourself acquainted with the code base, you could start by taking a look at the docs of MappingProcessor which give a high-level overview of the general architecture. Then check out the org.mapstruct.ap.model and org.mapstruct.ap.processor packages.
If you have any questions don’t hesitate to ask or StackOverflow, we are here to help!