Check out example codes for "camera pinch zoom unity". It will help you in understanding the concepts better.

Code Example 1

//I SUGGEST YOU WATCH THE VIDEO ON HOW IT WORKS
//https://www.youtube.com/watch?v=0G4vcH9N0gc&t=127s

using System.Collections;
using System.Collections.Generic;
using UnityEngine;

public class PanZoom : MonoBehaviour {
    Vector3 touchStart;
    public float zoomOutMin = 1;
    public float zoomOutMax = 8;
	
	// Update is called once per frame
	void Update () {
        if(Input.GetMouseButtonDown(0)){
            touchStart = Camera.main.ScreenToWorldPoint(Input.mousePosition);
        }
        if(Input.touchCount == 2){
            Touch touchZero = Input.GetTouch(0);
            Touch touchOne = Input.GetTouch(1);

            Vector2 touchZeroPrevPos = touchZero.position - touchZero.deltaPosition;
            Vector2 touchOnePrevPos = touchOne.position - touchOne.deltaPosition;

            float prevMagnitude = (touchZeroPrevPos - touchOnePrevPos).magnitude;
            float currentMagnitude = (touchZero.position - touchOne.position).magnitude;

            float difference = currentMagnitude - prevMagnitude;

            zoom(difference * 0.01f);
        }else if(Input.GetMouseButton(0)){
            Vector3 direction = touchStart - Camera.main.ScreenToWorldPoint(Input.mousePosition);
            Camera.main.transform.position += direction;
        }
        zoom(Input.GetAxis("Mouse ScrollWheel"));
	}

    void zoom(float increment){
        Camera.main.orthographicSize = Mathf.Clamp(Camera.main.orthographicSize - increment, zoomOutMin, zoomOutMax);
    }
}

Learn ReactJs, React Native from akashmittal.com