You'd definitively be interested by the ryg[1] series on the graphic pipeline. It goes in details about how bits get to the GPU and how the GPU handle them. The author writes that using a 3D graphics API is a prerequisite. To the contrary, the series itself is great to get a grasp on things and not feel completely alienated by 3D graphics APIs if you ever decide to use one in the future. It's fairly old (2011) but I still see it recommended as a relevant resource today.

[1] https://fgiesen.wordpress.com/2011/07/09/a-trip-through-the-...