The expression of the mesh in blender can be smoothed by setting a smooth shade on the object and then adjusting the angle with automatic smoothing.

 

However, in the case of an object that you want to keep a part such as a monkey's face flat, it may be a strange expression.

 

There are two patterns for sides and faces, so please set them to your liking.

🌸 Select an edge and adjust with a sharp edge

🌼 Select a surface and adjust it flat and smooth

I will put a reference link below.