Re: smoke detectors
Each building is different. In a duplex you usually have two separate systems, one for each side, not interconnected with the other side. Common areas may have double coverage, or no coverage, depending on where it is.
Large buildings will have a system designed by an electrical engineer. The smokes will be separate from the appliances (bells and lights)
In a hotel, the smokes will be monitored at a central station.
In an apartment building, it could be most anything, depending on the ratings of the fire partitions separating the apartments, and whether the building is sprinklered.