How does a fire alarm detect smoke?

Smoke detectors work in several ways.

A point to point smoke detector detects smoke when the beam projected by the transmitter is obscured in a measurable way and detected by the receiver.

A photoelectric detector sees smoke particles inside the detection chamber of the smoke detector because light inside the chamber is scattered in a measurable way.

An ionization smoke detector detects smoke because the smoke particles change the 'charge' on the detection plate inside the smoke detector.