Have you actually read the linked site beyond its name? The site is about fundamentals. Yes, it contains OpenGL code, but what it covers is the fundamentals of real time graphics, not just OpenGL API.

> Have you actually read the linked site beyond its name? The site is about fundamentals.

Is it? All the fundamentals are tied to how it's done in OpenGL with somewhat sparse explanations in between. And many "fundamentals" are literally something like

    #version 330 core
    struct Material {