It seems that kapt is the entry point, am I right?

This might help 🙂 https://proandroiddev.com/hello-world-of-annotation-processing-in-kotlin-3ec0290c1fdd

I think these articles are going to deep. Usually you need just Kapt to run AP, after that you can continue generate Java code without changes or your AP. Generation of Kotlin code is also possible, but not sure that it make a lot of sense to rewrite generator, you can just use existing one with Java and Kotlin sources because Kapt allows you to do that