Note:
  This feature is currently only available on Android,
  taking advantage of Android and Google Play Stores’
  dynamic feature modules to deliver the
  deferred components packaged as Android modules.
  Deferred code does not impact other platforms,
  which continue to build as normal with all deferred
  components and assets included at initial install time.

Also, note that this is an advanced feature.

Though modules can be defer loaded,
the entire application must be completely built and
uploaded as a single Android App Bundle.
Dispatching partial updates without re-uploading
new Android App Bundles for the entire application
is not supported.

Deferred loading is only performed when the app
is compiled to release or profile mode.
In debug mode, all deferred components are treated
as regular imports, so they are present
at launch and load immediately. Therefore,
debug builds can still hot reload.

For a deeper dive into the technical details of
how this feature works, see Deferred Components
on the Flutter wiki.