I installed
AC hardwired smoke detectors during the last heavy renovation. They are all wired together as part of the standard 3-conductor AC wiring scheme outlined in the installation instructions. When any one of them triggers, they all sound. I think this has become almost standard now.
Yeah, and when one of them is regularly falsing at 3am, only one or two nights a year, that design means
you can't tell which one falsed and needs replacing. :-)
I disconnected the interconnection wires on them all and sure enough, one of them falsed, just one, and I finally replaced it. :-)