There are at least three different definitions of "in the kernel":

- code that compiles to the single executable file that runs the OS (this can be limited to a mechanism for booting the system, a thread scheduler, basic memory allocator, but this may also contain some device drivers, file systems, etc)

- any code that compiles to anything that will run in kernel mode (many file systems and device drivers)

- the code that someone (say the people distributing what they call "the Linux kernel") call the kernel.

This count is about the third. I do not think it contains a kitchen sink, but it is close :-)

I've got a lot of .ko files in /lib/modules; that doesn't mean they aren't part of the kernel.