By "desirable motherboard" in this context I mean a standard ATX (or standard size variants) motherboard with a currently supported socket and chipset commonly available on the consumer market. To run Intel 13th or 14th gen, or Ryzen 7000 or 9000. I don't know if you can just buy an MSI or Asrock etc. board and expect to run Coreboot on them.

What's the advantage of coreboot? Soothes paranoia mainly. Both Intel and AMD platforms have little black boxes in them that run a separate little OS beneath Windows or Linux that has Ring 0 or similar low-level access to the hardware and could theoretically man in the middle anything done on the machine. Intel's is MINIX based, it's called the Intel Management Engine, and it genuinely is a little bit bile inducing reading what it has access to. AMD does have a simlar technology.

In terms of performance, system stability etc? Very little. Once the kernel is loaded and in control of the hardware the BIOS doesn't effect much AFAIK.

I'm not very familiar with it but I've not heard much about even AM4 boards being supported. I think of Coreboot (or it's completely binary blob free fork LibreBoot) and I think of either Purism or System76 and in both cases for their laptops.

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This kind of thing (the "main" operating system is built atop a secret basement full of god knows what) isn't restricted to x86 either. On a Raspberry Pi, Linux running on the ARM cores is a second class citizen to ThreadX running on the VideoCore processor.