To understand how MapStruct works internally you need to have understanding of Java annotation processor tool (JAPT)
Basically JAPT is a way to generate java source files during the compilation time. Its works on annotation and create new files based on that.
MapStruct is actually JAPT only which create new files based on annotations. In MapStrtuct you have to define a interface(With @Mapper annotation) which declares any required mapping methods. During compilation, MapStruct will generate an implementation of this interface. This implementation uses plain Java method invocations for mapping between source and target objects.
Note: Just remember one thing that MapStruct generates code at compile time.
Now we have baisc understanding of MapStruct so let's start playing with it >>