Houdini 16 introduced a new streamlined shader building workflow, called Material Networks. Shader building now works entirely at the VOP level, which is much easier for layering, maintaining shaders in the pipeline, and it is a much more intuitive workflow.
Have a look at this masterclass by Kai Stavginski, the senior technical director at SideFX. Kai covers everything you need to use the new shader building context and shows you how to build custom shaders with it.
The result of the masterclass ends up as a custom shader that has all the outputs that are required for mixing with other built-in shaders. The custom shader will be able to blend individual shaders for some nice results.
Kai also demonstrates the differences between the Principled Shader and the Principled Shader Core, the Classic Shader and benefit of cached code.
Towards the end of the masterclass, Kai also explains OpenGL tags to visualize shaders in the viewport.