What else makes the Hall Sensor work?

I see my speedo needle and mileage doesn’t work so I cleaned the Hall Sensor and it still is not working so I ordered a new one for $65.

My question is what makes this work as I only see the sensor that bolts onto the caliper and plugs into the speedo. There must be a magnet or other pick up item for the sensor to sense revolutions? I had another machine that was setup this way but needed a tiny magnet on the hub for the sensor to see revolutions .