The Wikipedia (almost certainly incompletely) lists 23 kernels and 43 kernel developers – and there are more in development. Most of the articles on the internet that try to explain kernels are either incomplete or incorrect or both. Understandable, since the complexities don’t lend themselves to easy-reading. Most of us don’t really need to look up rigorous texts since CAD-data-translation has little to do with the kernels themselves and more to how these are used by specific CAD application programs. See this 2-part discussion on the format-ladder for more.