Mining is done as an operation on the Steem blockchain. The data section can contain whatever the developer wants it to. Steem has its own format of how people can take actions.

Let's say you want to transfer some STEEM to Ned.

There is a specific format of a message that essentially says, "stellabelle transfers 1.000 STEEM to ned". The result of the message is that 1 STEEM is removed from your balance and added to Ned's.

Likewise there is a message that says, "Alice mined block X with nonce Y". The result of this message is that Alice gets placed in the queue to produce a block.