How would you instantiate something relative to the position of the previously instantiated object?

Question

So basically, for the past couple hours I have been trying to figure this out. Currently, what I have is some code that instantiates an object within a certain x and z range, 5-10 metres higher each time.

private GameObject platScript;
public GameObject[] platforms;
public float xRange = 50.0f;
public float yRange = 5f;
public float objectRange;
public float yMinRange = 5f;
public float yMaxRange = 10f;
public float zRange = 50.0f;
public float rotRange = 360.0f;

// Start is called before the first frame update
void Start()
{

}

// Update is called once per frame
void Update()
{
    if (Input.GetKeyDown(KeyCode.K))
    {
        PlacePlatform();
    }
}

void PlacePlatform()
{
    int platformsToSpawn = Random.Range(0, platforms.Length);
    yRange += Random.Range(yMinRange, yMaxRange);

    platforms[platformsToSpawn].transform.rotation = Quaternion.Euler(Random.Range(-rotRange, rotRange), rotRange, Random.Range(rotRange, rotRange));
    Vector3 RandomSpawnPos = new Vector3(Random.Range(-xRange, xRange), yRange, Random.Range(-zRange, zRange));

    GameObject platStan = Instantiate(platforms[platformsToSpawn], RandomSpawnPos, platforms[platformsToSpawn].transform.rotation);
    platScript = platStan.GetComponent<GameObject>();

    Vector3 platPos = new Vector3(platStan.transform.position.x + Random.Range(-10, 10), 0, platStan.transform.position.z + Random.Range(-10, 10));

    Instantiate(platforms[platformsToSpawn], platPos, platforms[platformsToSpawn].transform.rotation);
}

What this code is supposed to do is get the current x and z range of the instantiated object, add between -10 and 10 to it, and then spawn it within a radius of -10 to 10 relative to the last instantiated object, rather than anywhere in a -50 to 50 radius on the x and y axis.

I tried running this, but it doesn't seem to have any effect on whether the platforms spawn relative to the last instantiated object or not. They just spawn anywhere between -50 to 50 on the horizontal axis, going 5-10 metres higher each time.

I'm quite new to c#, having only picked it up about 2 weeks ago, so my code is probably quite a mess, and I apologise for that. Any advice on cleaning my code or how to get this to work would be Greatly appreciated. Many thanks.