If there's nothing readily available online, my first idea would be to skim over the source for a few different already existing filesystem drivers.

https://github.com/gotoco/dummyfs

Maybe this will help?

this is what people always suggest. read the code that is already there.

my experience has been that you quickly need to then learn how several other major parts of the kernel work which is non trivial and something i struggle with.

there is also no comments in the code which makes it even harder.

and the books that are there are not up to date with recent kernels AFAIK.