How to make different assets combine onto one another?

If anyone could tell me or show me a link on how to make it so that I can have two cubes or any other assets be joined together, as in they would kind of “snap” onto each other, and would be detachable in-game as need be? Thanks

Right click second cube, choose ‘attach to’, and use the ‘eye dropper’ ( top right in the dialog ) to choose the first cube.