This should really be titled "Introduction to WebGL". The GL programming model is very different from what's actually going on under the hood. GL it at best a faint reminder of how GPUs worked during the 90's.
This is by now also over a decade old but still gives a much better idea how current GPUs work:
FWIW OpenGL only mapped to how the hardware worked for a particular vendor (Nvidia) and a very short time (~GeForce 2, IIRC).
OpenGL was always a higher level API, it just is an API that provides functionality instead of being a framework (so you had to make your own). Even in SGI hardware a lot of features were done only in software and during that time on PCs most cards could barely do more than rasterization and transformations (and some not even that).
Even that wasn't really OpenGL anymore, even though some internal registers matched OpenGL directly.
The only HW that was truly OpenGL was Silicon Graphics IRIS workstations. IrisGL was released as open standard with some minor changed with the name OpenGL.
This is by now also over a decade old but still gives a much better idea how current GPUs work:
https://fgiesen.wordpress.com/2011/07/09/a-trip-through-the-...