In the 1.17 snapshot 21w10a, more shader types were added for resource packs to be able to edit. As the game’s OpenGL version was upgraded, more possibilities were added for customisable shaders. This is a repo which attempts to record and document each of the new core shader types, what they do, and general information about core shaders.
An explanation what the different matrix multiplications do can be found here. In Minecraft Model and View Matrix are combined into a ModelViewMatrix.
You can find an explanation of vanilla shaders in this document.
The currently WIP basics section can be found here