May i ask how to do the prefab bullet hits or collides with rocks then the rocks fall? i don't know how to do it. anyone please help me :(

You Could Say When The Bullet Hits The Rock, It Applys A Rigid Body To The Rock. Would Give More Detail, But Your Question Is Unclear.

You could animate the rock, simple drop animation, or if that area is hilly you could just activate its physics and it would roll down OR you could add force till it drops down. If you are using raycast, get the hit object and getcomponent of the script where is the function, which launches the animation. If you are using rigidbody colliders, just get the collider hit, and next is the same process as with raycast.

To get script and call some function on the run, see this example i gave answer for: Send Message on Trigger Enter? - Unity Answers