"integrate Kotlin files into a Java project without using IntelliJ" - compilation of a mixed project is not a trivial tasks, many things must be set up. A detailed description of this takes a chapter in a book.

That's the reason one should use build tools, be it Ant, Maven, or Gradle. The process of setting them up without an IDE is narrowly described in the docs https://kotlinlang.org/docs/tutorials/build-tools.html