Handler-Looper implementation in Android

Question

1. I have Activity with Handler (UI thread)
2. I start new Thread and make handler.post(new MyRunnable()) - (new work thread)

Android documentation s

Answer 1

Handler attached to UI thread.

Correct.

How android can run runnable in the same UI thread without new thread creation?

Any thread, including the main application ("UI") thread, can call post() on Handler (or on any View, for that matter).