Merge mining is mechanism that allows bitcoin miners to miner other blockchains/sidchains while mining the main bitcoin blockchain. Merge mining already exists and was first implemented by Satoshi in 2011 with Namecoin/BitDNS. Blind Merge Mining (BIP301) the is just a standardised version so bitcoin miners can miner multiple sidechain at once without needing to run sidechain/L2 Nodes.
Rootstock/RSK sidechain already enables merge mining, the only downside of RSK is that its owned by IOV labs and has federation of known members who can be captured by the state and steal your funds at any moment
I recommend reading this article on merge mining by Sergio Lerner to fully understand how it works in practice: https://medium.com/iovlabs-innovation-stories/merged-mining-and-decentralization-7c2590f928ed
