It is simply 3Core + Earth wire from a smoke alarm to the Test Switch.
The diffuilctly is finding and making a route to run the wire from the ceiling, down the wall and making it all look good.
I have it in the kitchen so it is on escape route (don't want to run into a closed room to silence alarms if a real fire), and becuase it is near my burning toast and I can silence alarms rest of house when I set off alarm near kicthen.
Aico also do a wireless switch.
You can mix and match wired and wireless (with Aico, dunno about others) so if you can't get a cable to everything, you can bridge the gaps with wireless - but I'd try hard to find a cable route before resorting to that.
In our flat I have two heads hardwired, another haed and the switch hardwired, and radio units in two of the heads to link the two islands together. When you press the test switch, you can hear one alarm go off quickly, then a few seconds later the other two go off.
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