Stratum jobs don’t contain the full list of transactions included in the block template. A miner only needs to construct a block header, which can be done without knowing the full template contents. Modern miners exhaust the 32-bit nonce in the block header quite fast and can then either update the timestamp in the header, roll the version a la overt ASICBoost, or change the so-called extranonce in the coinbase transaction, which causes the Merkle root to change. For this, miners need the coinbase transaction, information about the extranonce, and the Merkle branches to calculate a new Merkle root.

The list of Merkle branches in stratum jobs contains just the information required to calculate the Merkle root. To build the Merkle root, the coinbase transaction is hashes together with the first Merkle branch, the result is then hashed with the second Merkle branch, which is then again hashed with the third Merkle branch. The Merkle root is reached once all Merkle branches have been hashed together.