Struggling to understand how to write custom mod shaders

I've learned how to write basic JSON files that work with vanilla shader templates (like the spider eye or blur effects). However, I don't understand how to access uniform variables. For example, I want to create a blur that fades in smoothly over time. I'm also interested in how to stack or overlay shaders on top of each other — the way mods like Blur and Enchanted Visuals do it.

Could anyone explain the general approach or point me to relevant resources for Forge 1.20.1?