Create mask to see underneath an upper layer

Good people, my project requires a mask effect which creates the effect of looking deeper into an object. For example, the upper layer contains an engine and when the mask is applied, the pistons inside the engine are visible. Ideally, the mask should be interactive and dragable by the user.

I have figured out the code for the dragable mask on a mouse click but have not succeded so far in creating the final end result of looking underneath an upper layer through a mask.

Your inputs are higly appreciated 🙂