In addition to what @irvinewade said, I would add that both Intel and AMD have copyrighted reference designs for circuit boards using their processors, which every PC maker uses to some degree when designing their computers. Because Intel and AMD don’t allow PC makers to publicly release their schematics, there are no x86 PCs on the market with published schematics. However, you can occasionally find leaked board layouts for some models, and System76 will provide the schematics to their customers who ask for them and provide the serial number of their PC, but System76 (Clevo) doesn’t allow them to be published. Therefore there are some people who have been able to look at the schematics and board layouts for x86 PCs, but not the general public.
You need the schematics to understand how each wire is connected and the board layout to see where each component is placed on the circuit board. If you want to be able to repair a device or use a multimeter, oscilloscope or function generator to test components, then it helps to have the board layout. If you want to detect inserted spy chips, you need x-rays of the boards, so you can x-ray your own device and compare it with the published x-rays.
Here are the current computing devices with some published info on their hardware, that I know of off the top of my head:
- RaptorCS publishes the board layout (PDF with normal copyright) for its POWER9 Talos II board (but not for its other boards).
- OLIMEX publishes all sources for its devices including the CAD files under an Apache 2.0 license, including the TERES-I DIY Laptop with an Allwinner A64 processor.
- PINE64 publishes the schematics for its devices (PDF with normal copyright) on their wiki. I know that some people have obtained the board layout views of their devices, but they aren’t published, so I assume that PINE64 only shares them with a limited number of people.
- Purism publishes the schematics and board layout drawings (PDFs under the GPL 3.0) and x-rays (only for Birch so far, CC 4.0 license) for the Librem 5 (but can’t release anything for its x86 devices due to Intel’s copyright restrictions).
- There are a handful of open hardware SBCs with free/open source licenses: Arduino, Intel Galileo (gen 1 and 2), LittleBits CloudBit, BeagleBone, Olimex OlinuXino, ADI MinnowBoard Turbot.
- The Raspberry Pi is not open hardware, but it publishes the schematics and basic mechanical drawings (PDF with normal copyright).
If your concern is inserted spy chips, board layouts and x-rays may help you discover that, but I think that it is extremely unlikely that end-users (not servers and routers) have to worry about inserted spy chips for these reasons: 1, 2, 3