I heard about Hermes, this tool for react native is supposed to make the app faster by converting the JS code into bytecode (Android).
Now my question is: Does this mean that the index.android.bundle in the APK file will contain only bytecode instead of JS code ?
That is correct. One of the features of Hermes is that it reduces TTI by moving JS compilation from runtime to app package build time.
You can verify this by inspecting the contents of your APK.