Do you mean the small radiuses advised by paint manufacturers on the joints in windows and similar on shaker doors?
I don't really do them but a router bit with a tiny radius or a couple of swipes with a block plane will do.
As lofty says, cut into the material to add these line details, blade selection will determine how much material is removed. A thicker blade will obviously remove more, creating a bold line whilst a fine blade will remove much less creating a more feint subtle line.