How can I get triggered, when I place any shaped object in same shaped holder in Unity?

Hi Guys, I am a newbie in Unity. I am trying to make a 2D kids' game. will explain through an example. I have one placeholder, It can be any shape. for now, it's like "L Shape". Placeholder is our target.

there will be 4 squares randomly placed in the scene. out of 4 squares, only one square can move, This square should collect the other 3 squares, collected squares shape should be like the placeholder shape, here it should be L, The collected L shape should place it in the L shaped placeholder. Please help me to figure out this, How can I get now the collected shape exactly placed in the place holder? I have attached one image and video here, for getting a better understanding

https://imgur.com/a/Nb8cxv1

https://www.youtube.com/watch?v=vMpRTdQtdCM