Using the example from this question's answer, and following jtgrassie's recommendation to use https://xmrchain.com with "More Details" -> "Show JSON representation of tx", the tx extra field can be found in here.
EDIT: better instructions
Go to https://xmrchain.com
Enter the transaction hash you want to look up, e.g. the example here
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Is there an example of a transaction that makes use of tx_extra field? Is this public unencrypted data?
Every transaction makes use of tx extra as it places the tx public key in there. The data is public but can be encrypted (e.g. payment IDs are encrypted in extra).
...that is viewable in the monero block explorer?
Select a transaction in https://...
So what exactly the miner_tx_hash field in the block, and what is its relation to the miner_tx and other transactions in the block?
miner_tx_hash is just that - the transaction hash of the miner transaction. The miner transaction is the transaction that pays out the reward for mining the block. The inputs to this tx are type gen as they literally generate ...