Random point:

Vector3 position = new Vector3(
    Random.Range(-10f, 10f),
    0f,
    Random.Range(-10f, 10f)
);

Spawn:

Instantiate(
    prefab,
    position,
    Quaternion.identity
);

Random point inside a sphere:

Vector3 point =
    Random.insideUnitSphere *
    radius;