First of all, create the prefab of the necessary turret parts from the prefabs folder.

Result of combining prefabs

Next, you should add the following scripts to the created prefab:

• Turret - main script which controls the turret rotation settings and fire tick variable.
• Target Finder - script to find possible targets to start attacking.
• Child of Firing Event script - main script to determine how the turret fires.

In the Turret script, we pass a game object that will rotate:

Also we pass the projectile prefab as reference of a game object.

In the Target Finder script, we pass a game object that we use as a pivot to find targets:

By default, target tag for a game object we want to target is Enemy.

In the child of Firing Event script, we pass a turret gun game object. For example, we use Simple Fire script:

Next, you should add the Gun script to the turret gun prefab. It holds the muzzle object to determine transform position for instantiate projectile prefab:

In other cases, you probably need to add some kind of animations - fire or barrel rotation scripts.