sanapping specific face object to other specific face object

Hi I've been looking for a good tutorial about snapping some specific parts of an object, but unsuccesful.
hera an example I need to build an cube tower, but the cubes have diferents desings & adjust only on side by side or above from other, etc. How can I snap just one of the faces of this cubes & not all the cube?
I found the way of drag the cubes in unity itself, but I don't know how to make the snapping in a particular face of the cubes.
Thanks for your help.