One or more packages that contain the platform-specific
implementation code. The app-facing package calls into
these packages—they aren’t included into an app,
unless they contain platform-specific functionality
accessible to the end user.

The package that glues the app-facing packing
to the platform package(s). This package declares an
interface that any platform package must implement to
support the app-facing package. Having a single package
that defines this interface ensures that all platform
packages implement the same functionality in a uniform way.

Endorsed federated plugin

Ideally, when adding a platform implementation to
a federated plugin, you will coordinate with the package
author to include your implementation.
In this way, the original author endorses your
implementation.