I have disassembled two motor circuits that use a Hall sensor. One is a cooling fan blower and the other is a 230 VAC motor.
The blower has a brushless motor and has its Hall sensor on the stator board with the coils. The rotor is a solid magnet cylinder encompassing the whole stator.
The motor had its Hall sensor (SOT23, unrecognizable marking) facing a solid disc.
How does it detect turns (or calculate speed) if the magnet is a solid body?