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.

Apparently most fires do not cause the AC power to fail, at least not on the circuit powering the detectors. By the time the wiring has been burned through, the people should be long evacuated. Batteries go dead much more often than AC failures coincident with unexpected fire events.

I note that the built-in Lithium battery powered smoke detectors have enough energy to run for the rated 10-year life span, at which time they start beeping to demand their retirement from service.