I have an object with smooth faces, on which I am applying a Boolean diff modifier in order to create some holes
in it, but the resulting face are now not smooth. is there some way to make it better ?
Try to model the holes manually. I think to use boolean for this is not a good technique. Try to keep the quads!
The boolean operation resulted in a bunch of triangles. One solution is to model the hoel indvidually, the other is to select all the edges and choose dissolve limited. Might flatten things out a bit for you.