Membrane.Pipeline.handle_prepared_to_playing

You're seeing just the callback handle_prepared_to_playing, go back to Membrane.Pipeline module for more information.

handle_prepared_to_playing(context, state)

handle_prepared_to_playing(
  context :: Membrane.Pipeline.CallbackContext.PlaybackChange.t(),
  state :: state_t()
) :: callback_return_t()

Callback invoked when pipeline is in :playing state, i.e. all its children are in this state.