Ask HN: How OS developers find specifications needed to write drivers?

As someone interested in OS development, I read articles, source code and tutorials on the topic. I have created a Linux module, I have an idea of how a bootable "hello world" ISO is created etc.

Recently I looked into an open-source OS written from scratch, with support for some peripherals, networking, graphics and I asked myself whether I would know how to implement those. I picked rtl8168d (that was one of the implemented drivers) and tried to find the specification - without success.

My question is: how does an OS developer that started a toy OS and wants to add basic networking/graphics/mouse know what specification to implement to get it working on as much hardware as possible? Once he knows that, how does he find the specs?

I hope it's not by reading other OS's source code and trying to do the same thing (whether it's correct or not).