which blockchain is the preprod anchor stored in ?

When I create an anchor via the pre-prod system I receive info about its storage in the blockchain, eg:

block hash = 28a8dab39ab2b76ecd72a53c564541664d1b6b4f1e856a392d67b304f6bce9dd
tx_id = d7a77cc586f4c862a9780d86e3483d07734e2303e412b6ec5a7778c6206d8fa7
OP_RETURN = f3cef70364a8fb391e5184af5f704ac5042b65d25faae3ad886a72f7c6433535

but I cannot find these data in the BTC blockchain. I imagine that, being a pre-prod system, my anchors are not being written to the real BTC blockchain. If that is true, which one are they written to ?

I also noticed that the proof field in the proof receipt is empty, and the merkle root is equal to the target hash. eg :

{
"targetHash": "f3cef70364a8fb391e5184af5f704ac5042b65d25faae3ad886a72f7c6433535",
"merkleRoot": "f3cef70364a8fb391e5184af5f704ac5042b65d25faae3ad886a72f7c6433535",
"proof": [],
"anchors": [
{
"sourceId": "d7a77cc586f4c862a9780d86e3483d07734e2303e412b6ec5a7778c6206d8fa7",
"type": "BTCOpReturn"
}
],
"type": "ChainpointSHA256v2",
"@context": "https://w3id.org/chainpoint/v2"
}

Is this also a side-effect of the pre-prod environment ? What does a "production" proof look like.

thank you for your help

Nicholas Quaine, ADEC SAS