The back panels are either made of glass (NP1/⅔/3a/4a) or plastic (NP2a), underneath the panel isn’t the actual hardware, but rather a design layer, which consists of thin plastic and metal parts as well as housing the glyph lights.
You could try getting a cheap, malfunctioning Nothing Phone from ebay or wherever and experiment on it. You could try to remove the back panel and replace those design elements with your own. I don’t know if common 3d printers could print such thin elements.
If you want a more in-depth idea, I suggest going to Nothing’s YT channel and check out their design videos about recent and previous phones: Typically you see various iterations and prototypes and get a rough idea how it works.
