Merged mining, also known as Auxiliary Proof-of-Work (AuxPoW), allows multiple cryptocurrencies to be mined simultaneously. It enables a miner to use their computational power to secure multiple blockchains without dividing their hashrate.
In merged mining, the work done on the parent blockchain (e.g., Bitcoin) is also used to validate transactions and create blocks on the auxiliary blockchain (e.g., CORE).
Merged mining is implemented by embedding metadata from the auxiliary blockchain into the coinbase transaction of the parent blockchain's blocks. This metadata typically includes block hashes and other relevant information.
The merkle root for the witness data is in an OP_RETURN output.