The thermal power plant uses a boiler to heat the water in the pipe of the inner wall of the boiler to form superheated steam with high temperature and high pressure. The superheated steam then turns the steam turbine and the generator is coaxial with the steam. the turbine turns to produce electricity.
Hydroelectric power plants use the momentum formed by water when there is a drop to turn a hydraulic turbine, and a generator coaxial with the hydraulic turbine rotates to produce electricity.
Thermal power plants are polluting, but not hydroelectricity.