Comment by IshKebab

I'm not totally sure, but I would imagine those stock distros actually have dedicated packages for Raspberry Pi kernel images.

See this for example: https://www.phoronix.com/news/Raspberry-Pi-5-Ethernet-Linux

If you look at the patch series, it's directly adding information about the address of the ethernet device. That's the sort of thing that would be discovered automatically in the x86 world. It wouldn't need to be hard-coded into the kernel for each individual board that is supported.