unity – how to make a destructible object?

You do not necessarily have to make the object itself destructible. Instead, you can create a number of separate models of "pieces" of your object.

If the object explodes, you can then draw an explosion effect, delete / hide the original object, and then create the various pieces projected randomly. Each is a new distinct and unique object.

If the object breaks in a more natural way, you can make a variation of the same idea, but that parts will all appear at their original location if the object has been assembled. Assuming physics is active, the pieces will fall and fall as they normally would.