During a Falcon 9 launch you hear the commands 'throttle up' and 'throttle down'. Who or what performs 'throttle up' and 'throttle down' (i.e. astronaut or computer)?
Ascent to orbit is all computer controlled. All Falcon 9 flights prior to the Demo-2 mission were uncrewed, so no astronaut was present to operate a throttle control.
You're not hearing commands but rather reports. Onboard computers determine that it's time to throttle down (usually based on elapsed time), issue the command to the engine control hardware, the engine throttles down, the telemetry system sends a message to the ground indicating that the engine has throttled down, and a human says "throttle-down".