Two styles: (1) speed and (2) torque.

The speed system provides more motor assist the faster you pedal, common on cheaper ebikes and lower end conversions.

The torque system provides more motor assist the harder you pedal. This the big feature of the TSDZ2 mid drive conversion.

The torque sensor system is always in the bottom bracket, because it has to sense how hard the rider is pedalling. It can't do that at the hub because of gearing and the motor drive (unless it is a front wheel drive motor). There are torque sensing bottom brackets available and controllers that will interface with them and standard motor control units.