walk

Returns a sequence of paths for visiting this directory and all its contents.

fun Path.walk(vararg options: PathWalkOption): Sequence<Path>(source)

import java.nio.file.Paths
import kotlin.io.path.*

fun main() {
    val root = Paths.get("src")

    root.walk()              // visits root and all its sub‑directories
        .filter { it.isRegularFile() }  // keep only regular files
        .forEach { println(it) }       // print each file path
}

Source