Any libraries with changed code

The application’s main library

The libraries from the main library leading
to affected libraries

The source code from those libraries is compiled into
kernel files and sent to the mobile device’s Dart VM.

The Dart VM re-loads all libraries from the new kernel file.
So far no code is re-executed.

The hot reload mechanism then causes the Flutter framework
to trigger a rebuild/re-layout/repaint of all existing
widgets and render objects.