Function build_default_syncing_engine 

pub fn build_default_syncing_engine<Block, Client, Net>(
    config: DefaultSyncingEngineConfig<'_, Block, Client, Net>,
) -> Result<(SyncingService<Block>, Net::NotificationProtocolConfig), Error>
where
    Block: BlockT,
    Client: HeaderBackend<Block> + BlockBackend<Block> + HeaderMetadata<Block, Error = Error> + ProofProvider<Block> + Send + Sync + 'static,
    Net: NetworkBackend<Block, <Block as BlockT>::Hash>,

Build default syncing engine using build_default_block_downloader and build_polkadot_syncing_strategy internally.