Help me understand Merge Mining SYS w/ BTC?

self.SysCoin1m ago
Do all BTC mining nodes automatically run SYS mining or interact with the SYS network? Or does each individual BTC miner have to specify that they would also like to mine SYS? Is the SYS hash different than the BTC hash? If so, what happens if the BTC hash is mined and it moves on to the next block, but the SYS hash isn't found? If they are the same hash, what happens if the hash is found by a BTC miner that's not connected to the SYS network? ​ Thanks!