It doesn't really need specific configuration. Most linux kernels will boot if not given an initramfs and just a root=<whatever> command line. The main thing is whether enough kernel modules are compiled in to find and read the disk and filesystem.
Yes, it requires specific configuration. The reason most distributions of Linux use them is it allows you to dynamically discover and load kernel modules at boot time, and provide a fallback for if mounting the disk fails.
A "BSD from scratch" project would end up closer to something along the lines of "Debian from scratch" or "Redhat from scratch". You would be performing an exercise in bootstrapping, and little else.
It came as a shock to me to learn that initramfs is mostly optional. It can be skipped, and you can boot straight into userland.
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