Requirements for smoke alarms in dwellings have changed considerably over the decades, and the differences between a house wired in different eras are substantial.
The oldest homes have no wired alarms at all, only battery units added later by occupants. Somewhat newer construction has hardwired alarms with no interconnection, so an alarm in the basement sounds only in the basement. Current requirements generally call for hardwired, interconnected alarms with battery backup, located in each sleeping room, outside each sleeping area, and on every level.
Interconnection is the part worth understanding. Traditional interconnected alarms use a three conductor cable between units. Two conductors provide power, and the third is a signalling conductor, usually orange or red. When any alarm detects smoke, it puts a voltage on that conductor, and every other alarm on the loop responds by sounding.
This is why an alarm in an attached garage or a basement can wake someone upstairs, which is the entire point. Most fatal residential fires happen at night, and detection in an unoccupied part of the house is only useful if the sound reaches the occupied part.
Two practical notes follow.
Mixing brands on an interconnect loop is generally not supported. The signalling arrangement is not standardised across manufacturers, and units that appear identical may not communicate. When replacing alarms, replacing all of them with matched units is the reliable approach.
Alarms have an expiry date. The sensing element degrades, and manufacturers specify a service life, commonly ten years from the date of manufacture printed on the back. An alarm past that date may still respond to the test button while no longer detecting smoke reliably. The test button verifies the sounder and the electronics, not the sensor.
Wireless interconnection is now available and solves the problem of adding interconnected alarms to a house that has no signalling conductor between locations, without opening walls.
Location rules matter too. Alarms too close to kitchens and bathrooms produce nuisance activations, and an alarm that gets disabled because it keeps going off protects nobody.
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