Dell 15 7573 with 4k display wrong or missing GPU? Confused!

Not sure why the NVIDIA GPU is detected as an Unknown Device in the BIOS -- maybe try downloading and installing the latest BIOS firmware update if that's just a cosmetic bug that has since been fixed? -- but if it's properly identified in Device Manager, then you definitely have an NVIDIA GPU.  Having multiple graphics cards in the same system has been pretty commonplace on laptops for quite a while now.  In the most common design found in most laptops, most likely including yours, the Intel GPU is the only one physically wired to the display outputs, and it is always active.  The NVIDIA GPU is used only when the drivers detect that an application that is currently running would benefit from its additional horsepower, or you can go into the NVIDIA Control Panel to force it to run with certain applications if necessary, though it's usually not.  But even when it's active, it operates as a render-only device which then passes completed video frames to the Intel GPU to output them to the appropriate display(s).  This design means that when you don't need the extra graphics horsepower, you benefit from the reduced battery consumption of only having the Intel GPU active, but the NVIDIA GPU can still transparently be enabled on-demand when performance is the priority.  And just as a fun fact, part of the reason that basically every laptop now has an Intel GPU is that Intel now builds the GPU into the CPU, and most if not all laptop systems use CPU models that have the GPU element included.