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.
The trick to getting clean edges is as follows
So lets say we have a cube as our low poly object and another cube with sculpted detail at the edges of the cube as our high poly mesh. First mark every hard edge as a seam and unwrap. Make sure to increase the padding in the unwrap settings to something like 0.01. The bake the normal map in the software of your choice (if you have the money don't harm you mental psyche and use a proper program for baking like marmoset or substance). Now for the most important thing make sure the normal map is set to Non-Color. It will look terrible at the edges if this is forgotten.
Some extra things that could fix baking problems
Have a material on the high poly object. Make sure the cage object circumferences every part of the high poly object. Make sure that all normals are pointing in the correct direction. Make sure the shading is correct in blender before baking. Shading can be corrected with Normals > Average > Face Area, Normals > Merge and so on (Alt+N).
Some baking info
For baking one needs a detailed object(the high poly object) an optimized version of that(the low poly object) as well as a cage that is made out of the low poly object and scaled up to circumferences the high poly object. When baking in blender the high poly object needs a default material however when using marmoset for example the material can be ignored. The low poly version needs to be UV unwraped but the high poly object doesn't need to be UV unwraped. No matter high extreme the detail, everything can be baked, the question is does it make sense. Baking is essential in the games industry.