help to understand collision detection

hello!
what i want do understand is how to detect collision without colliders…
what iàm doing now is this

    vector3 playerPos = new Vector3 (playerPosition);
    Vector3 planePos = new Vector3 (planePosition);
    
    float distance = Vector3.Distance(playerPos,planePos);
    
    if (distance <= 1)
    {
       // do something
    }

it’s working for a simple things, like a not resized plane, a plane it’s exactly what i’m using…
as showing in the following image a want a way to understand when player is going off that a specific plane, like colliders, i’ll regulate also the y position of the character by the plane position so you can understand that it should be more accurate as possible.

there is a way to create somithing like colliders?

Is it a grid?
If not, are the areas always rectangular/square?
Are they always aligned?

You could make use of mesh.bounds, convert that to world space. Then test if your char.position is within that space.

If you went with colliders, it could be triggers for areas. Or normal colliders and detect what you are standing on.