How do I prevent hand movements from counting as steps on Apple Watch Series 9?

Use the information here to calibrate your watch for accurate step count. It learns your movement behavior to calculate steps correctly.

Calibrate your Apple Watch for improved Workout and Activity accuracy - Apple Support

Rongo5 wrote:

I have ‏Apple Watch Series 9 (GPS). When I do some hand’s movements the watch’s pedometer count it as steps. Is there anything to do to change it?

There really isn't a way to prevent it entirely. I always remove my watch when knitting or crocheting as both will show up as steps.

Wear it on your ankle❓

The Watch mainly uses the movement of your watch arm to count your steps, even if you started a workout.