The error in your code is that you are making a list in every iteration of the loop. You should make the list first and then add every item from the loop to it!

fun convertRoomClass(courses: List<Course>) : List<AnotherCourseClass> {
    val newList = mutableListOf<AnotherCourseClass>()
    courses.forEach {
        newList += AnotherCourseClass(it.pathID, it.pathName, it.pathDescription)
    }
    return newList
}

A better solution is to use the map function

fun convertRoomClass(courses: List<Course>) = courses.map {
   AnotherCourseClass(it.pathID, it. pathName, it.pathDescription)
}

You might be looking for Kotlin Map

Example:

course.map { Courses(it.pathID, it.pathName,it.pathDescription) }

You're getting the list with only on object, cause the function listOf(crs) returns a list of all objects that are passed as a parameters. Saying the same thing in Java you're doing something like this:

for (course: Courses) {
    Course course = new Course(...);
    List<Course> list = new ArrayList<>();
    list.add(course);
    return list;
}

As you can see the it created new list with a single object per iteration. What you're trying to achieve, can be done with operator map{...} which simply transforms every object in the initial list using code passed inside map and returns list of transformed objects

course.map{ Courses(...) }

Also, I've noticed that you're using the !! operator when creating a Courses object. Probably because the Course can have nullable name, while Courses can't. I'm considering this as a bad practice, cause in this case you're saying

Please throw an Exception if the name is null.

I think that a much better approach is to provide an alternative, like:

val name = course.name ?: "default", saying

Please use name or "default" if the name is null.

or skip objects without name, or any other approach that suits your situation.