The next step up from that is to then render meshes using some form of projection (isometric or perspective). You'll need to figure out how to actually map the drawing to the screen at that point. (In high school we just used the built-in 2D methods in Java. Some of my classmates then went on to shading and textures, but I didn't quite get that far.)
Of course it has more to do with raytracing than with 3D projections (which is most often used for real time 3D rendering). If you want to learn about 3D projections, first you should understand what linear algebra has to do with projecting a 3D model on a 2D screen. This is probably a good start.
You should DEFINITELY write your own matrix library to learn the underlying principles, but once you want your renderer to actually do something with reasonable speed you'll pretty quickly be replacing your homegrown matrix code with something like Eigen that handles SIMD for you.
Note that when you're building test cases, matrix multiplication is not commutative.
My high school teacher literally just pointed me at the Wikipedia article and said go:
https://en.wikipedia.org/wiki/Matrix_multiplication
The next step up from that is to then render meshes using some form of projection (isometric or perspective). You'll need to figure out how to actually map the drawing to the screen at that point. (In high school we just used the built-in 2D methods in Java. Some of my classmates then went on to shading and textures, but I didn't quite get that far.)