Step-1:  First we have to open a NEW project, Create project UNITY 3D.

Step-2:  Next step is to import the UNITY assets there are unity assets which should be imported.

Here are names and links to import them.

•  Environment

https://assetstore.unity.com/packages/3d/environments/japanese-city-modular-pack...

•   Shooting Gun

https://assetstore.unity.com/packages/3d/props/guns/desert-eagle-w-custom-skin-1...

• Enemy

https://assetstore.unity.com/packages/tools/ai/enemy-ai-163967

Step-3:  Next step is to place all of them to create our shooter game by placing the environment in the scene and adjust the lights in the environment.

Step-4:  Now place the Gun and Enemy in the game scene.

Step-5:   Now, Enjoy the game.