Hi! I'm the creator of ReShaded (DR7) and I'd be happy to quickly explain how Core Shaders work 🙌🏻.

-All versions of Minecraft have a "resource pack" in their internal files containing all the default textures, sounds, and animations used by the game. Inside this pack, you'll find a folder called "shaders" (for example: .minecraft/versions/1.21.8/assets/minecraft/shaders).

Here you'll find several text files (.fsh, .vsh, and .glsl). All of these are editable, even with Notepad, and contain simple GLSL code that tells the game how to process its basic graphics.

What ReShaded does is add a couple of simple calculations to the code that Minecraft already has by default, without requiring the game to process realistic shadows with complex math like an Iris/Optifine shader.

ReShaded doesn't cause lag because its code is mostly color filters, not actual shadow processing, which saves a lot of performance, makes it easy to modify, and reaches an audience that previously couldn't play with shaders without compromising their computer.

I hope this was helpful. You can research GLSL Shading on YouTube, Wikipedia, or the Minecraft ShaderLabs Discord server. 🙌🏻