There are no "extended blocks". Blocks are the same as before; they only contain the block header and the transaction. What changed was the transactions. The transactions have two different formats: legacy, and segwit. When a segwit capable nodes receives a block, it will receive the block with transactions in the segwit format. When a non-segwit node receives a block, it will receive the transactions in the legacy format.

When you run a segwit capable node, using the getrawtransaction or gettransaction commands will give you all of the details of the transaction, including the witnesses.