How to reduce color variation and gradient steps in an image or remove pixel noise(Posterize, Surface blur, Median)

1) Open an image in Photoshop.

Posterize

Posterize limits the tonal levels of an image, making colors appear more distinct.

2) Go to Posterize in Adjustments from the Image menu at the top.

3) Set the levels value and confirm

4) You can see that the color variation of the image has decreased and the gradient steps have been lowered.

Surface blur
5) Go to Surface blur in Blur from the Filter menu at the top.

6) Set the Radius and Threshold values ​​and check.

7) You can see that the noise in the image has been removed and the blur effect has been added.

Median
8) Go to Median in Noise from the Filter menu at the top.

9) Set the Radius value and check.

10) You can see that the noise has been removed by mixing the pixel brightness of the image.