how to make airplane hit a buillding and destroy the airplane(its part of my game)

Questions & Answers
1

i need help how to make airplane that i maded with a cube hit a building that i maded with a cube too and be destroyed(i dont know how to make the airplane hit the building too)

2

Well basically both the cubes need colliders. Then you make a script which you attach to the airplane where you check if collision occurs. If so, you can remove the airplane and replace it with an explosion prefab.

I suggest you look in the manual how to add colliders.
For the scripting part this can be a good reference : Collider.OnCollisionEnter

Explosion prefab examples can be found in certain asset packages included in unity.