If you’re interested in this topic and want a great way to get started you should check out the Broken Thorns tutorial. There are a few others but this single resource is very consice and is interspliced with a good amount of theory as well as all of the implementation.
The project seems to be rather defunct. There a lot of pull requests for fixes that have all been ignored. There haven’t been commits since September of 2015.
Even if slightly, or even extremely out of date, the material in this is a good primer for the kind of stuff you should be looking into if you want to write an OS. Stuff like grub, theory about os design, and other topics seem to be well covered in this document. It would be nice if it was updated but I think it still holds some value even if it isn’t up to date.