How coroutines switch back to the main thread.

MainActivity.kt

class MainActivity : AppCompatActivity() {

    override fun onCreate(savedInstanceState: Bundle?) {
      super.onCreate(savedInstanceState)
      setContentView(R.layout.activity_main)

      launch(context = UI) {
        
        val result = async { loadString() }.await()
        Log.d("TAG", 
            "Post execution thread:"+Thread.currentThread().name)
        textView.text = result   
      }
    }

    private suspend fun loadString() : String {
       Log.d("TAG", "Execution thread: "+Thread.currentThread().name)
       delay(1000L)
       return "Some loaded string..."
   }
}

Which version of coroutines did you use to have the launch method in the Activity?