I might be retarded, but explain how flow through turbine works

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I might be retarded, but explain how flow through turbine works pls
>incompressible working fluid
>before the turbine there's volume flow 1, with velocity x
>after the turbine the velocity must have been reduced as power is extracted from the turbine, so velocity must be less, but the volume flow must also be the same or otherwise the water cant get out of the way fast enough
???