Spawn objects (or randomise their position) inside an area defined by a 2D collider.

RandomiseInsideCollider.cs

using UnityEngine;

public class RandomiseInsideCollider : MonoBehaviour
{
    public Collider2D AreaPolygonCollider;
    Bounds Bounds;
    Vector3 BoundX1, BoundX2, BoundY1, BoundY2;
    Vector3 randomPos;

    public Transform go;

    void Start()
    {
        Bounds = AreaPolygonCollider.bounds;
        BoundX1 = Bounds.min;
        BoundY2 = Bounds.max;
        BoundX2 = new Vector3(Bounds.max.x, Bounds.min.y);
        BoundY1 = new Vector3(Bounds.min.x, Bounds.max.y);
        RandomizePosition();
    }

    void RandomizePosition()
    {
        randomPos = new Vector3(Random.Range(BoundX1.x, BoundX2.x), Random.Range(BoundX2.y, BoundY2.y), 0.0f);
        if (Physics2D.OverlapPoint(randomPos))
            go.position = randomPos; // alternatively, instantiate object here
        else RandomizePosition();
    }
}