Check out example codes for "unity gizmo draw line". It will help you in understanding the concepts better.

Code Example 1

using UnityEngine;
using System.Collections;public class ExampleClass : MonoBehaviour
{
    public Transform target;    void OnDrawGizmosSelected()
    {
        if (target != null)
        {
            // Draws a blue line from this transform to the target
            Gizmos.color = Color.blue;
            Gizmos.DrawLine(transform.position, target.position);
        }
    }
}

Code Example 2

// you need the LineRenderer component in your object

var line = gameObject.AddComponent<LineRenderer>();

line.SetPosition(0, startingPoint);
line.SetPosition(1, middlePoint);
line.SetPosition(2, endPoint);

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