For example i have 2 models. One low poly and second high poly. Low poly has hard edges. High poly after subdivision mod will have smooth edges at same position where lowpoly has hard edges. After baking "high on low" in Substance painter can normal map (from high poly) smooth those hard edges on lowpoly ? Or u need to smooth those hard edges on low poly manually with bevel, smooth shading etc. Can normal map do fake smoothing ? Or normal map can only add fake bump ?
Yes, a normal map can do "fake smoothing". A normal map changes how light interacts with the surface, specifically the angle at which it reflect. So a normal map can make light reflect off a hard edge as if it was a rounded edge.
The limit is that it will not change the silhouette of your object. So, if you look at a hard edge from the side, you can still tell it's a hard edge with a normal map. However, if you can model and texture well, these problems can often be mostly dealt with.
Just in case you don't know as well, Blender can 'bake' normals from high poly to low poly objects as well, in case that would help you.