However, I need pointers to get started.
> You won't be spoon-fed, and are expected to have read the manuals and other documentations...
I read a lot of FreeBSD and NetBSD documentation to get to the point of compiling my own kernels, but I don't think I ever read about the equivalent concept of Linux cpio/initramfs for BSD. My minimal images use a UFS filesystem.
Here, after checking https://mfsbsd.vx.sk/ and https://github.com/mmatuska/mfsbsd/blob/master/scripts/mdini... I think mfsbsd is just a using tmpfs so it may not exactly the same thing as initramfs, that allows booting linux from a bzImage + initrd
I'll keep searching, it's not super high priority at the moment, but it's something I'd like to do with (Free|Net)BSD.