We've seen all these ingenious 1
The basic concept is quite simple: Like the above background images, these cases show how your iPhone looks without a back. You will see the motherboard, the battery, and all the other fancy stuff. And of course, they fit perfectly with the innards of your phone.
Each case is made of thermoplastic polyurethane, that's the full name of what you've heard a hundred million times: TPU. (Read: They're made of plastic.) However, it looks like they have a bit more Oomph on the corners, which should protect your device in the event of a fall.
Two versions of each case exist: color and x-ray. iFixit describes it as "a color version to display the internal components as if you had just raised the screen, and an x-ray version that lets you look right through the back of your phone to see what's inside." So you can recognize the guts of your phone.
These cases are now available on certain iPhone models only: XR, XS, XS Max, X, 8, 8 Plus, 7, 7 Plus, 6s, 6s Plus, 6 and 6 Plus. Basically all iPhones that were made in the last half decade. Maybe someday we'll see them for some Android phones as well.
You are looking for cost reasons for each case after ten digits. This is damnably competitive even in a crowded phone case market (even for a TPU case), and the fact that it looks super cool is just the icing on the cake.