The way Lighthouse works is there is a LED flash that denotes the beginning of the scan pass for the laser. Then the laser scans across the room in a known pattern taking a known amount of time.
The photosensor mounted on the headset sees the LED flash denoting the beginning of the scan, then begins a high resolution timer, waiting for the laser to hit it. Based on the time the laser hits the sensor, it can then calculate its direction relative to the watchtower, multiple sensor hits result in a position via triangulation.
The more sensors on the object, the more precise and reliable the position information. This is fine, since the sensors are dirt cheap to implement (they're just a photosensor and a timer, not a camera).
This visualization makes it easier to understand what it is doing:
https://www.youtube.com/watch?time_continue=4&v=J54dotTt7k0
The way to think of Lighthouse is that it works kind of like an old-fashioned SNES CRT light gun (like you use in Duck Hunt), but in 3 dimensions instead of 2.
https://en.wikipedia.org/wiki/Light_gun
