I know CONTRIBUTING.md but there is somewhere a more detailed description of the internals of pyvista with e.g. UML or another general explanation of the code base to find your way around more quickly. The Docs is primarily for users, I'm more interested in 'offal' to be able to contribute. A TODO would also be practical.
The best UML diagram or understanding of PyVista will come from looking at VTK. PyVista directly inherits VTK types and simply has a bunch of functions to make accessing VTK meshes and filters more streamlined. The VTK textbook was really helpful for me when I started learning VTK