Check out example codes for "unity clamp rotation". It will help you in understanding the concepts better.

Code Example 1

float rotationX = 0;
float rotationY = 0;
// you might also have some rotation speed variable
void Update() {
   rotationX += Input.GetAxis("Vertical") * Time.deltaTime;
   rotationX = Mathf.Clamp(rotationX, minRotationX, maxRotationX);
   rotationY += Input.GetAxis("Horizontal" * Time.deltaTime;
   transform.rotation = Quaternion.Euler(rotationX, rotationY, 0);
 }

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