linux was initially designed for just x86. that's the real reason it's complicated in this case: each architecture defines it's own system call table and dispatch, for example, instead of having a common one amongst architectures.

other kernels (like NetBSD) where designed with being portable across architectures in mind and so have much cleaner code paths (typically just one system call table and the bare minimum arch specific code to link it all together).

minix is okay but it's not really going to give you a grip on a real work kernel. "design and implementation of 4.4BSD-lite" is a really good book for this: most BSDs are very similar to this design still in a lot of ways, and many of the ideas described (e.g., VFS) are also used in linux.