Passable one sided collision platforms

The easiest way to replicate that in Unity would be to use mesh colliders. That is, put quads at the top of the boxes; you don't need renderers on these quads, necessarily. You could do it like Smash Bros., where the characters phase through the bottoms of platforms like Shadowcat, but can stand on top. Mesh colliders only cause collisions to happen in the direction they face, so point the faces up.

And if you stand on the white one, and hold the "down" key for a little while, turn the mesh collider into a trigger for a frame. ;-)

Just use Unity's PlatformEffector2D Component

https://docs.unity3d.com/Manual/class-PlatformEffector2D.html

This helped me. On the top of the platform I've put a reguler 2D collider and underneath a trigger collider. Then I've put this script on the platform object using UnityEngine; using System.Collections.Generic;

 public class BottomPassObject : MonoBehaviour {
 
     private List<Collider2D> colliders = new List<Collider2D>();
     private Collider2D bottomTrigger;
     private int enterCount = 0;
 
     void Start () {
         foreach(Collider2D col in this.GetComponents<Collider2D>()) {
             if (col.isTrigger)
                 bottomTrigger = col;
             else
                 colliders.Add(col);
         }
     }
 
     void OnTriggerEnter2D(Collider2D other) {
         if (other.tag.Equals("character")) {
             this.enterCount++;
             foreach (Collider2D col in this.colliders)
                 col.enabled = false;
         }
     }
 
     void OnTriggerExit2D(Collider2D other) {
         if (other.tag.Equals("character")) {
             this.enterCount--;
             if (this.enterCount == 0) {
                 foreach (Collider2D col in this.colliders)
                     col.enabled = true;
             }
         }
     }
 }

Explanation: - That character tag check is because my character (for other reasons) has another child object with collider that I need to be ignored. - the list of colliders is because some of my platforms have more colliders - The enterCount is because my character also has more colliders on him

Hope this helps