With UV sterilizer cases or bags, is it ok to stack items on top  of each other?

For example, you laid your phone on the bottom of the case. And then placed your face mask on top of it. If you were to put them side by side rather than stacking on on top of the other, would it make any difference?

I ask because you see some pictures of these units being sold where items are just next to one another. In other pictures, you'll see items covering or touching other items. With larger bags, there might be say a sweater on the bottom with a shirt on top of it, partially covering it. And other items, like jewelry, etc on top of these.

So would the UV rays penetrate through the items to disinfect what's underneath or on top? Or would the rays be blocked, even partially by having items stacked on each other? And thus, stacking items would defeat the purpose because the top or underside wouldn't be disinfected.